Epoxidation of oleic acid with oxygen in the presence of benzaldehyde using cobalt(II) tetraphenylporphyrin (Co2+-TPP) as catalyst was studied. The results show that high yield of epoxidized oleic acid is obtained by the liquid phase epoxidation of oleic acid in the presence of benzaldehyde using Co2+-TPP as catalyst, which does not catalyze the decomposition of perbenzoic acid, which is one of the oxidation reagents for epoxidation and is formed by the oxidation of benzaldehyde. The oleic acid can be easily epoxidized in this system. The reaction mechanism was proposed. The experimental and theoretical results indicate that the epoxidized oleic acid is formed by a series of free-radical reaction steps. The rate-determining steps were experimentally identified, and the rate equation of epoxidation was obtained. The factors affecting the rate of epoxidation of oleic acid were also obtained.