Mechanical properties of composers would be obtained mainly by photopolymerization by visible light irradiation. Protopolymerization behavior of two commercial compomers was investigated by FT-IR spectral analysis and differential photocalorimetry(DPC). The results obtained were as follow: 1. From FT-IR analysis, it was found that the compomer products were photocured mainly by radical polymerizations of multifunctional methacrylate monomers. Final conversion values of the polymerizations after exposure to visible light for 60 sec were 67% for Dyract and 59% for Compoglass. 2. Upon exposure the compomers exhibited autoacceleration and autodeceleration, which are usually observed in bulk polymerizations of multifunctional monomers. 3. Dyract showed higher rate of polymerization and conversion of double bond than Compoglass.