Abstract: The CVD kinetics of pyrocarbon was investigated via the in-situ magnetic suspending balance，and the gaseous products  were analyzed via the out-line gas chromatography coupled with the mass spectrometry (GC/MS) during the pyrolysis of a propylene/argon mixture in the range of 800～1200℃. The kinetic results indicate that pyrocarbon is formed with an apparent activation energy of 137 kJ/mol±25 kJ/mol controlled by chemical kinetic reaction from 800℃ to 1000℃，Whereas，above 1000℃，the deposition behavior is representative of the mass transformation and gas phased nucleation. The dependences of pressure and residence time on deposition rate show that pyrocarbon is obtained after a series of gas phase reactions and solid surface reactions. The key carbon-formed species  controled by  each mechanism of the deposition process are different. A simplified reaction model describing the kinetic deposition process of pyrocarbon CVD is derived on the basis of the above considerations.