Composites combining polystyrene (PS) and thermally reduced graphene with different size of sheets were fabricated by melting blending. Composites added large and small size of graphene (graphene mixture) exhibited a higher electronic conductivity. To reduce the weight of composites, PS/graphene composite foams were prepared by scCO(2) foaming process. As a result, composites included different size of graphene showed a more uniform bubbles, higher cell density and smaller cell diameter because of synergy in graphene sheets. The foams with graphene mixture also exhibited high cell density in high saturation temperature due to their higher viscosity and glass transition temperature analyzed by high-pressure rheometer and differential scanning calorimeter. For conductivity of foams, the introduced bubbles exhibited a good enhancement of electrical conductivity and percolation threshold decreased from 0.34 vol.% to 0.16 vol.%. It could be found that conductivity increased by raising pressure and decreased with the increase temperature.