The radical copolymerization of maleimide (MI) and ethyl alpha-propylacrylate was performed using 1,1,2,2-tetraphenyl-1,2-bis(trimethylsilyloxy) ethane (TPSE) as initiator. The whole copolymerization process might be divided into two stages: in the first stage, the copolymerization was carried out on the common radical mechanism, the molecular weight of the copolymer increased rapidly in much lower conversion (< 85%), and did not depend on the polymerization time and conversion; in the second stags, molecular weight of the copolymer increased linearly with the conversion and the polymerization time. It was found, however, when the conversion was higher than a certain value, for example, more than 36%, the molecular weight of the copolymer was nearly unchangeable with the polymerization time and the molecular weight distribution was widened. The effect of reaction conditions on copolymerization was discussed and the reactivity ratios were calculated by the Kelen-Tudos method, the values were r(MI) = 0.13 +/- 0.03, r(EPA) = 0.58 +/- 0.06 for TPSE system and r(MI) = 0.12 +/- 0.03, r(EPA) = 0.52 +/- 0.06 for AIBN system.