A series of well-defined block copolymers of poly(ethylene oxide-b-styrene)s (PEO-b-PS)s with narrow polydispersity were synthesized by the following two-step approach: living anionic polymerization of ethylene oxide (EO) with sodium 4-oxy-2,2,6,6-tetramethy-1-piper-idinyloxy (TEMPONa) as initiator to yield PEOn with a TEMPO moiety at chain end, was followed by a stable free radical polymerization (SFRP) of styrene (St) to give a block copolymer. In the process of anionic polymerization, TEMPONa could induce living anionic opening-polymerization of EO at low temperature under which the stable niroxyl radical at the end of the PEO chain could be not destroyed at all. The TEMPO moiety in the resulting PEOn acts as a radical trapper in quantitative efficiency and St can be designed for polymerization for good control of the SFRP. Such a polymerization proceeded with a living radical mechanism. However, PEOn should be more sterically hindered and possess lower diffusion ability and thus be less efficient at trapping propagating chain radicals, resulting in enhanced polymerization rate.