The epoxidation of high cis-butadiene rubbers (BR) with monoperoxy phthalic acid was successfully performed in reactive processing equipment of the HAKKE mixer at room temperature. The effects of such reaction conditions as the concentration of peroxy acid and reaction time on the epoxidation level and the efficiency of monoperoxy phthalic acid were investigated. The structure of epoxidized BR was characterized by H-1 NMR and Fourier transform infrared spectrometer. Differential scanning calorimeter analysis revealed that the glass transition temperature of epoxidized BR had a progressive increase with epoxidation level.