Recently, there is a high level of interest among chemists, physicists and materials scientists in using fillers of nanometer scale for preparing composite materials with exceptional properties. The incorporation of low loading levels of nanofillers such as silicon carbide (SiCp) and alumina (Al2O3) nanoparticles with large surface areas as well as carbon nanofibers (CNFs) with high aspect ratios into polymers improves their mechanical performances considerably. This research project was mainly focused on the fabrication, structural, thermal and mechanical characterization of polymer composites filled with ceramic nanoparticles and carbon nanofibers. The matrices of nanocomposites used were thermoplastic polyolefin (TPO), non-polar high-density polyethylene (HDPE), polar polyamide 6 (PA6) and their blends toughened with unmaleated or maleated poly(styrene-ethylene-butylene-styrene) (SEBS or SEBS-g-MA). The structure and morphology of polymer nanocomposites and hybrids were examined by means of X-ray diffraction (XRD), polarizing optical microscopy (POM) and scanning electron microscopy (SEM). The thermal behaviors of the nanocomposites were studied using differential scanning calorimetry (DSC), heat deflection temperature (HDT) and thermogravimetric analysis (TGA). Tensile, impact and essential work of fracture (EWF) methods were employed to characterize the mechanical properties of nanocomposites. From the experimental results obtained, the structure-property relationship of polymer nanocomposites was discussed. For the composites based on PP/SEBS-g-MA blends, both thermoplastic-rich (TPO) and elastomer-rich (ETPO) matrices were selected. The former consisted of 30% elastomer (SEBS-g-MA) and 70% PP, and the latter composed of 70% elastomer and 30% PP. Silicon carbide nanoparticles and carbon nanofibers were employed to reinforce TPO while only SiCp were used for ETPO. The two composite systems were fabricated by melt extrusion and injection molding techniques. DSC and HDT results showed that SiCp and CNFs acted as nucleating agents and improved the thermal stability of TPO blend. Tensile and impact results revealed that SiCp additions stiffened and strengthened TPO at the expenses of tensile ductility and impact strength. Moreover, CNF additions simultaneously improved the Young's modulus, yield strength and impact strength of TPO blends. EWF measurements also indicated the beneficial effect of CNF additions on improving the fracture toughness of TPO blend. In contrast, ETPO/SiCp nanocomposites were found to exhibit lower stiffness and strength but enhanced impact strength compared with ETPO blend. For polyamide-based system, binary PA6/SiCp and ternary PA6/SEBS-g-MA/ SiCp nanocomposites were fabricated through one-step melt mixing process. DSC results showed that SiCp served as effective nucleating agents for PA6 and PA6/SEBS-g-MA blend. Mechanical tests showed that the Young's modulus and tensile strength of PA6 increased with increasing SiCp content up to 5 wt% at the expenses of tensile elongation and impact strength. The incorporation of 20 wt% SEBS-g-MA into PA6/SiCp nanocomposites resulted in enhanced ductility and impact strength but poorer Young's modulus and tensile strength. Therefore, it is necessary to maintain the stiffness-and-toughness balance for the PA6/SEBS-g-MA/SiCp hybrid composites to achieve optimum mechanical properties. Tensile EWF results indicated that SEBS-g-MA elastomers were beneficial in improving essential work of nanocomposites while SiCp impaired the fracture toughness of nanocomposites investigated. Finally, maleated HDPE (mPE) nanocomposites filled with SiCp were also prepared by melt compounding and injection molding. XRD, POM and HDT results showed that SiCp additions reduced the crystallite thickness and spherulite size, and improved the thermal stability of mPE blend. Tensile and impact tests revealed that SiCp additions enhanced the Young's modulus and yield strength but reduced the impact toughness. Accordingly, SEM fractography was used to reveal the failure deformation of nanocomposites after impact test. The low impact strength of mPE/SiCp nanocomposites can be attributed to the absence of particle cavitation and matrix fibrillation. The impact fracture deformation of such nanocomposites was discussed in details. To restore the impact strength of mPE/SiCp composites, 10-30 wt% SEBS were added to form mPE/SEBS/SiCp hybrids. Finally, the effects of alumina nanoparticle additions on thermal and mechanical properties of HDPE were also studied. Alumina nanoparticles were treated with silane agent to improve their interfacial bonding with the polymer matrix. DSC, HDT and TGA results showed that Al2O3 nanoparticles acted as nucleating sites for HDPE crystals and improved the thermal stability of HDPE matrix irrespective of surface treatment. Tensile and impact tests demonstrated that surface modification of alumina nanoparticles led to enhancement of stiffness, strength and toughness of the nanocomposites.