Abstract: Block copolymers were synthesized by conventional free radical polymerization in two steps by using polyfunctional chain transfer agent. In the first step，free radical polymerization of methyl methacrylate was carried out in the presence of a chain transfer agent，1，2-ethanedithiol. Hence，PMMA with residual thiol groups at the chain end were obtained at appropriate monomer conversions. Thus prepared PMMA with residual thiol groups at the chain end was subsequently used in the second step as a macro-chain transfer agent，where conventional free radical polymerization of styrene was carried out yielding block copolymers of PMMA with polystyrene successfully.