Copolymers of 4'Methacryloyloxybenzalacetophenone-nButylmethacrylate (4'MB-nBuMA) were synthesized and the effect of the benzalacetophenone group to the sensitivities of the copolymers were investigated. The copolymers are hardned by irradiation with UV light because of the formation of cyclobutane linkage. The reaction rate of the photodimerization was found to be second order with the concentration of the benzalacetophenone group in the film state. 4'MB- n BuMA copolymeres were spectrally sensitized by conventional triplet sensitizers, Michler's ketone, p -nitroaniline and 5- nitroacenaphthene. The efficiency of the sensitization increases with increasing the concentration of the benzalacetophenone group in the copolymer.