The epoxidation of unsaturated fatty acid methyl esters (FAMEs) by peroxyacetic acid generated in situ from hydrogen peroxide and acetic acid was studied in the presence of SO3H-functional Bronsted acidic ionic liquid (IL) [C3SO3HMIM][HSO4] as catalyst. The effects of hydrogen peroxide/ethylenic unsaturation ratio, acetic acid concentration, IL concentration, recycling of the IL catalyst, and temperature on the conversion to oxirane were studied. The kinetics and thermodynamics of unsaturated FAMEs epoxidation and the kinetics of oxirane cleavage of the epoxidized FAMEs by acetic acid were also studied. The conversion of ethylenic unsaturation group to oxirane, the reaction rate of the conversion to oxirane, and the rate of hydrolysis (oxirane cleavage) were higher by using the IL catalyst.