Polycarbosilane (PCS) is synthesized from polydimethylsilane (PDMS) by thermal condensation reaction in an inert atmosphere. In the new process described in this article, CO2 at high pressure (2-10 bar) was introduced into the system before the condensation reaction of PDMS. Si-C backbone formation process was accelerated at 340-360oC, and brown "PCS" liquid was obtained at 380oC. Polymer recovery increased from 60% (in a pressure vessel with a N-2 atmosphere) to >95% in CO2 atmosphere at 10 bar. The apparent viscosity also increased at high CO2 pressures. Spectrum analysis indicates the existence of the Si-CH2-Si, Si-O-Si bridges, and Si-H bonds in the heat-treated product. The viscous liquid "PCS" obtained at 380oC is stable up to 200oC. However, at temperatures of 200-600oC, the volatilization of macromolecular species proceeded, and the resulting ceramic yield at 1000oC was 29 mass%. In the CO2 treatment, product solidified at 420oC and ceramic yield increased to 56%. The volatilization of macromolecules, however, proceeded in two steps at 200-400oC and 600-800oC. When the product was heated beyond 400oC, it probably contained infinite PCS network with shorter chains.