The polycarbosilane (PCS), which is the precursor of SiC fiber, was synthesized under high pressure by thermal decomposition of polydimethylsilane. The composition, structure, and properties of the PCS were characterized by the measurements of softening point, elemental analysis, IR, GPC, NMR, TG-DTG-DTA, XRD, and oxidative reaction activity, respectively. Structure model of the PCS was therefore inferred. The results showed that the PCS was the polymer with a Si-C backbone with M-n about 1587. IR and NMR showed the presence of SiC4 and SiC3H structure units containing Si-CH3, Si-CH2-Si, and Si-H groups. The ratio between H in C-H bond and H in Si-H bond was about 8.84 with SiC3H/SiC4 and about 0.51 from H-1 NMR and Si-29 NMR, respectively. Elemental analysis gave an empirical formula of SiC1.87H7.13O0.03. TG analysis showed that the ceramic yield of the PCS at 1200 degrees C in a N-2 flow was about 78.9%. beta-SiC microcrystal could be obtained when PCS was pyrolyzed at 1250 degrees C with the crystal size about 37.5 angstrom. Compared with the PCS with similar softening point synthesized under normal pressure, the PCS synthesized under high pressure had approximate elemental composition, higher Si-H bond content and reaction activity, higher molecular weight, and higher ceramic yield, but lower ratio of SiC3H and SiC4. (c) 2005 Wiley Periodicals, Inc.