Graphene oxide (GO) acts as an insulator but when reduced to reduced graphene oxide (RGO), it has high heat storage capacity. This can result in lowering the phase change temperature when embedded in a material such as expanded polystyrene (EPS), thus making it a good heat-resistance material. In this study, the thermal and the mechanical effects of GO and RGOs over EPS were analysed. GO was synthesized using modified Hummer’s method, while the reduction process was taken through different routes using three different reductants: hydrazine (RGO-HZ), ascorbic acid (RGO-AA) and the extract of Amaranthus hybridus (RGO-AH). The same content of graphite and its derivatives were separately and chemically bonded together with EPS forming composites (i.e., graphite composite, GO composite, RGO-HZ composite, RGO-AA composite and RGO-AH composite) and cast on a mould at room temperature to form a dog-bone shape. Fourier transform infrared spectroscopy, differential thermal analysis, scanning electron microscopy and mechanical tests were conducted on ordinary EPS and on each of the composite samples. The results showed a significant enhancement of thermal and mechanical properties of the composites over ordinary EPS.