Fe-Zn double metal cyanide (DMC) complexes are relatively cheap and effective catalysts for the ring-opening polymerization of propylene oxide．Polyoxypropylene diol oligomer is used as the initiator． Polyoxypropylene diols having Mn within the range of about 3000 to 12000 are prepared with Fe-Zn DMC catalyst under mild condition．The polymer is characterized by GPC and VPO．Effect of polymerization condition on molecular weight(MW) and molecular weight distribution(MWD) is investigated．The MW of polymer may be controlled by monomer／initiator molar ratio(M/I)．It is assumed that each activated catalyst site is related with many different molecules during polymerization．Polymerization proceeded at 80℃ and 60℃ gives polymers with a bimodal MWD(at least two kinds of active centers) and a unimodal MWD

respectively．When polymerization proceeds at temperatures lower than 65℃

the active center associated with Fe(CN)64- may not act because of its low activity．Two types of anion Fe(CN)63- and Fe(CN)64- coexisting in Fe-Zn DMC catalysts

which possibly result in the bimodal MWD

have been proved by IR spectra and X-ray diffraction．