In order to obtain the relationships between the conversion and the number average degree of polymerization and those between the conversion and theresidence time, the solution polymerization of methyl methacrylate (MMA) by azobisisobtyronitrile (AIBN) in benzene was carried out in a continuously stirred tank reactor (CSTR), using benzenethiol as the chain transfer agent.It was found that the conversion increased as the benzenethiol concentration was increasedin the range of 8x10-4 to 3.2x10-8 (mol/l) and that the index of molecular weight distribution (Q=Pw/Pn) decreased as the benzenethiol concentration was increased.