The method uB3LYP/6-31g+(d) was used to simulate the reaction of photochemical oxidation of olefins such as ethylene with ortho-ClNO₂Ph, para- ClNO₂Ph in the triplet state. It is found that photooxidation of an olefin occurs in two stages. In the first step, the complex formation of the nitro compound with the olefin has place and in the second step its further transformation to nitrosocompound and ethylene oxide occurs. The activation energies for the first reaction of orto-ClNO₂Ph with ethylene was 25.24 kcal mol^-1 and for the second reaction of para-ClNO₂Ph was 36.40 kcal mol^-1.  Conformation of the transition states was verified by IRC recovering and by the existence of the only negative frequency.