Abstract: A process for producing polycarbosilane partly containing siloxane bonds, which comprises adding 0.01 to 15% by weight of polyborosiloxane having phenyl groups in at least a part of the side chains of Si with its skeletal structure being composed of B, Si and O to a polysilane having the structure ##STR1## wherein n is at least 3, and R.sub.1 and R.sub.2, independently from each other, represent methyl, ethyl, phenyl or hydrogen; and heating the polymer mixture in an atmosphere inert to the reaction thereby to polymerize it. The polycarbosilane is converted into silicon carbide fibers by first preparing a spinning dope; subjecting the dope to a treatment of rendering it infusible under tension or under no tension and firing the treated dope in a vacuum or in an inert atmosphere.