A resin composition consists of a resin mixture, a light diffusing agent and a fluorescent-whitening agent. The resin mixture consists of 50˜99.9 wt % of polystyrene (PS) and 0.1˜50 wt % of polycarbonate (PC). The light diffusing agent is 0.1˜20 wt % of the resin mixture. The fluorescent-whitening agent is 0.00001˜0.02 wt % of the resin mixture. Therefore, the present invention has low cost because of PS and also possesses high thermal resistance and high impact resistance because of PC. Resin plates made from the resin composition, such as diffusion plates, have improved thermal and impact resistances.