Abstract: DSC results of poly(butadiene-b-dimethylsiloxane) diblock copolymers(PB-PDMS) with defined composition and molecular weight and narrow molecular weight distribution, synthesized by anionic sequential polymerization, showed that T_g of both blocks did not change with molecular weight and composition of the block copolymers, indicating a phase separation of the blocks. The crystallization behavior of PDMS block, however, was found to be effected by the composition and molecular weight of the block copolymers. TEM graphs and DSC results showed a clear relationship between crystallization behavior of PDMS and the morphology of PB-PDMS.