The reaction mechanism of 7-methoxy-5-ethenyl-octanitrile oxide cycloadditions have been studied with the molecular orbital AM1 method and transition state theory. The result showed that the titled compound generates the two stereoisomers( P_a and P_s) via transition states TS_a and TS_s respectively. The stereoselectivity was determined by the ratio of the rate constants of two reactions. The rate ratio for anti and syn is 2.5;i.e. anti/syn≈70∶30,which is in good agreement with the experiment productivity ratio(anti /syn=58∶42). The determining factors of the rate ratio come from the joint contribution of the activation enthalpy and activation entropy.