Poly(dimethylsiloxane-b-urethane) was prepared by reacting bis-(aminopropyl)-polydimethylsiloxane oligomer having aminopropyl terminal group with isocyanate terminated polyurethane prepolymers using tin octalate as catalyst at 5$0^{circ}C$, for 8 hrs. The copolymers formed by urea linkage were identified by IR and NMR spectroscopy, and the molecular weight of the copolymer was determined by GPC. It was revealed that dimethylsiloxane block unit in copolymer is retained its original length during the copolymerization. The mechanical properties such as elongation, tensile strength, and tan$delta$ of the copolymers containing various contents of dimethylsiloxane segment were investigated. Increasing the composition ratio of dimethylsiloxane block unit in the copolymer, the tensile strength was decreased but the elongation was increased. And also, the morphology to the film of copolymers with various contents of siloxane block unit was examined by scanning electron microscope.