In ternary composite systems, it is widely accepted that the impact and the tensile behaviors can be optimized by incorporating both elastomer and short glass fiber (SGF). However, in order to achieve property synergism of these multicomponent systems, the structure-property relationships knowledge is usually required. The mechanical properties are, in fact, not only controlled by the amount of additional phases added, but also the structure of the fiber-matrix interface morphology. The aims of this research work are to gain a deep understanding on the effect of morphologies of ternary composites on mechanical properties and their fracture mechanisms. Two main types of ternary composites have been prepared for investigation: 1) Polypropylene/SEBS-rubber/Short-Glass-Fibre (PP/SEBS/SGF) Ternary Composites 2) High-Density-Polyethylene/Polybutadiene-rubber/Short-Glass-Fibre (HDPE/PB-g-MA/SGF) Ternary Composites Structural and morphological tests have been carried out on the PP/SEBS/SGF ternary blends. The mechanical properties of the blends were studied by tensile test, Charpy impact test and fracture toughness measurement. The micro-structural features, fracture mechanisms, glass fibre length and orientation, etc., were examined with the help of optical microscopy (OM) and scanning electron microscopy (SEM). By controlling the fiber surface to be in direct contact with the rubber or the thermoplastic matrix (i.e. PP or HDPE), two different types of morphologies have been obtained by employing the functional group maleic anhydride (MA) as the interface controlling agent. The main morphologies developed in the resulting ternary composites are: (i) A portion of elastomer encapsulates the glass fibers surface while the remaining portion is dispersed finely in the thermoplastic matrix. These systems are referred to as the encapsulated structure in this work (ii) All glass fibers and the elastomers are evenly dispersed in the matrix. These systems are referred to as the separate dispersion structure in this work It is observed that the mechanical behaviors of ternary composites having the two types of microstructures are distinct, irrespective of the polymer matrix (i.e. PP or HDPE) used. Composites with separate dispersion structure exhibit higher impact toughness, higher yield strength but much smaller elongation at break than that of the encapsulated structure. The essential work of fracture (EWF) technique was used to determine the fracture toughness characteristics of the PP/SEBS/SGF hybrid composites. Similar to the conventional mechanical tests, composites with encapsulation structure have noticeably higher extensibility, determined by higher value of non-essential plastic work (;wp) while the separate dispersion structure composites are characterized by higher fracture toughness, denoted by the higher value of specific essential fracture work (we). However, as the fibre content increases from 10 wt% to 20 wt%, the properties between the two morphologies are similar since the degree of matrix plastic deformation was constrained by increasing the glass fibre content. Microscopic evaluations as well as four point bending test were performed to study the deformation mechanisms of the PP/SEBS/SGF composites. The results of fractographic analysis indicate that the dominant fracture mechanism of both structures is massive shear bands. Besides, remarkable interfacial debonding and mild rubber cavitations were observed for the composites with encapsulated structure, which would be the cause of their excellent ductility and fair toughness.