Hydrosilylation was employed for the preparation of polymethylsilicone-(polyalkylene oxide) block copolymers. As a basic test, simple diethoxyhydromethylsilane was allowed to react with simple unsaturated compounds such as allyl acetate, allyl acrylate, allyl methacrylate, etc. In this reaction, Kastard's catalyst [Pt-(CH2=CHMe2Si)2O] was more effective than Speier's catalyst (H2PtCl6·nH2O). On the basis of these results, polymethylsiloxane containing Si-H bonds instead of simple diethoxyhydromethylsilane were allowed to react with (polyalkylene oxide) allyl ethers in the presence of Kastard's catalyst. It was found that the copolymers obtained from the reaction with ω-methyl (polyethylene oxide) allyl ethers have narrower molecular distribution than those with (polyalkylene oxide) diallyl ethers, even though the average molecular weights were lower.