Polydimethylsilane(PS) was synthesized by the reaction of dimethyldichlorosilane with potassium and sodium alloy(50:50) in the cosolvent of xylene and THF(8:2) for 16hrs at final temp. 13$0^{circ}C$, the yield was 83%. But the yield was increased to 92% when 0.04% naphthalene of K-Naalloy was added to the reaction mixture. Polycarbosilane(PCS) was prepared by pyrolysis of polydimethylsilane and the yield was 34-36% when reaction was carried out at 320-45$0^{circ}C$ under nitrogen atmosphare without catalyst, but the yield was increased to 62-67% when 0.5-1% of borosilane was added as catalyst. They were identified by means of IR, UV. NMR. and elemental analysis and the molecular weight was determined by GPC.