In this paper, the kinetics of gel formation in polyurethane acrylate initiated by UV-radiation was investigated with the help of IR spectra, in which benzophenone/benzoin ethyl ether was a mixed photosensitizer. The reaction degrees of double-bonds changing at 1408 cm,-1, of acrylates in IR were detected in the different UV exposure time. The relationship between reaction degree P,C=C, and radiation time was established as follows: P,C=C,=0.325 t,0.135,. After a critical geloint,p the reaction degrees of double-bonds existed: P,C=C,(gel),>,P,C=C,>,P,C=C,(sol) and besides therewas a distributional relation between gel and sol fractions in the whole processing.The critical gel point was obtained with the extrapolation method, and its experimental gel point and the theoretical modeling one were quite close to each other. The experimental curves of P,C=C, P,C=C,(gel) and P,C=C,(sol)functions related to gel fractions were in fairly good agreement with their theoretical modeling ones in both shape and tendency.