A process for preparing organosilicon compounds comprising polycarbosilane which comprises pyrolyzing polysilane such as polydimethylsilane as the starting material and simultaneously polycondensing the products produced by the pyrolysis, characterized by, while maintaining the pressure at 2-10 Kg/cm² in the system, separating gases produced by the pyrolysis and polycondensation into non-condensed gases and a liquid by the use of a partial condenser and then continuously removing the separated gases from the system simultaneously with returning the separated liquid to the reaction system, thereby to obtain organosilicon compounds suitable as material for silicon carbide fibers.