Polycarbosilane was synthesized from the pyrolysis of polydimethylsilane, which is dechlorinated from dimethyldichlorosilane. The pyrolysis temperature was varied at 280-480$^{circ}C$ for 2 hours. FT-IR, FT-NMR and GPC measurement were studied on the residue inside the reaction crucible and viscous solution inside the cooling zone respectively. From the viscous solution in the cooling zone at 420$^{circ}C$, the synthesis of polycarbosilane wa well detected.