The boron carbide ceramic precusors-poly carborane(PCB) are synthesized by using pdibromobenzene(PDBB), boron trifluoride ether solution and magesium(Mg) as the raw materials. Carbon fiber(CF) is dipped in the solution of PCB and methyl benzene and is pyrolysised at high temperature, and the CF coated by boron carbide is obtained. The properties of the thermal oxidation resistance of the coated CF are studied.