The photopolymerization efficiency and surface hardness of composite resin containing 1,2-phenylpropanedione (PD) and diacetyl (DA) as photoinitiators were studied by IR and Vickers hardness and the results were compared with that of camphorquinone (CQ). Relative photopolymerization efficiency of the photoinitiators increased in the order of DA < CQ < PD. Vickers hardness of composite resin containing the photoinitiators increased in the order of CQ < PD < DA. Thus, PD is a new visible light photoinitiator for dental composite resin with higher photopolymerization efficiency and surface hardness than that of CQ. Mechanical properties such as Vickers hardness, diametral tensile strength, and flexural strength of the experimental resin composite prepared by addition of the photosensitizer into a resin of bis-GMA improved with increasing the photosensitizer content and irradiation time. The resin composite of bis-GMA containing DA or PD shows better mechanical properties than that of CQ.