The polycarbosilane (PCS) precursor for SiC fibers was synthesized in an autoclave from poly djmethylsilane (PDMS)．The influence of reaction temperature on the Si—H bond content,degree of branching,Si-Si bond content,molecular weight and distribution,softening point and yield of the PCS was investigated．The results showed that,with raising the reaction temperature the weight average molecular weight,degree of branching and softening point increased,the molecular weight distribution broadened and the Si—Sj bond content decreased．The Si一H bond content and PCS yield increased with temperature in the range below 460℃ and decreased over 460℃ The conversion process included initially the formation of PCS by thermal decomposition of PDMS and then the increase in molecular weight by condensation among the PCS molecules．An medium molecular weight peak appeared while the reaction temperature was over 450℃,and the PCS was stable with low Si—Si bond contents in air at room ternperature.