Ab initio UMP2(full) method has been used to study the mechanism for the reaction of C_2H_3 radical with O_2 on the potential energy surface. Energies have been calculated at Gaussian-3(G3) level. In this paper, we have optimized the geometry configuration of reactants, productions, intermediates and transition states. We have also calculated the activation energies of elementary reactions along the reaction pathways. The results suggest that the reaction mechanism of this reaction involves the formation of three-member, fourmember and five-member rings and produces different reaction resultants respectively. According to the calculated activation energies, the CH_2O and CHO are the main productions. At the same time, the reaction may form CH_3 + CO_2, CH_2CO_2 + H, C_2H_2 + O_2H and COHCOH + H, etc. The probabilities gradually decrease. The reaction heats gained in our study along the four pathways (C_2H_3 + O_2 →CH_2O + CHO, C_2H_3 + O_2 → CH_3 + CO_2, C_2H_3 + O_2 → C_2H_2 + O_2H and C_2H_3 + O_2 → OHCOH + H) are in good agreement with those of experiments.

Key words: ETHYLENE, FREE RADICAL REACTION, TRANSITION STATE THEORY, OXYGEN, REACTION MECHANISM