Carbosilane was Prepared by Pyrolysis of Polysilane under nitrogen atmosphere. The effects of Pyrolysis temperature and N2 flow velocity on carbosilane yield were studied. Thc pyrolysis of polysilane was carried out at the temperature range of 320~43$0^{circ}C$ with different N2 flow velocity. The maximum yield of 85% was obtained at 40$0^{circ}C$. The pyrolysis product was clear when the pyrolysis was carried out with N2 flow velocity below 265ml/min, and it was turbid with N2 flow velocity above 300ml/min. The structure of the pyrolysis product was identified as a carbosilane by infrared spectroscopic analysis. The average molecular weight of the carbosilanes was estimated to be 350 by gas chromatograpy-mass spectroscopy(GC-Mass) and gel permeation chromatography(GPC) Carbosilane contains Si-H groups in the backbone as confirmed by infrared spectroscopic analysis. The unit structure of the carbosilane prepared in this study was determined to be