This article presents an investigation of the modulation bandwidths of quantum-dot (QD) light-emitting diodes (QLEDs). The QLEDs used in our study are red-emissive CdSe/ZnS QLEDs, which have a structure of indium tin oxide (ITO)/poly(3.4-ethylene-dioxythiophene) polystyrene sulfonate (PEDOT:PSS)/TFB/QD/ZnO/Al and an emitting area of 2 or 4 mm2. We find that at a small injection current (below ~10 mA), the effects of the resistance-capacitance (RC) time constant and the carrier lifetime on the bandwidths of the QLEDs are comparable, while at a large injection current, the bandwidths are mainly determined by the carrier lifetime. The response time of the QDs is not a limiting factor. The bandwidths of the QLEDs increase with the injection current and are eventually limited by the damage threshold current of the devices. At the same injection current, the QLED that has a smaller emitting area provides a larger current density, and thus exhibits a larger bandwidth. At an injection current of 28 mA, the 2-mm2 QLED provides a bandwidth of 11.4 MHz and a luminance value of 156 000 cd/m2, and the 4-mm2 QLED provides a bandwidth of 8.2 MHz and a luminance value of 97 000 cd/m2. Our investigation provides a guideline for QLED-bandwidth optimization and useful information for the further development of QLEDs for lighting, display, and communication applications.