Copolymerized phenolic resin modified bamboo fiber composites(PR-BF) were prepared by a two step process i.e. firstly the bamboo fiber was immersed in an ultrasound irradiated bath of proper water solution of copolymerized phenolic resin and then undergone a heat treatment. The PR-BF composite was characterized by FTIR and its thermal property was examined by means of TG and DTG. The results show that bamboo fiber had excellent absorbability of phenolic resin; the phenolic resin is grafted on to bamboo fiber by copolymerization reaction; the thermal stability of the PR-BF composite increased dramatically with the increasing PR solid content in the solution; and the carbon residue of PR-BF reached a climax of 37.75% for the carburization at 900℃. From the FTIR analysis results it follows that in company with the thermal decomposition of PR-BF, a novel substitution product of benzene might form due to the copolymerization of BF with PR, which results in better thermal stability for PR-BF rather than the merely BF, and better resistance to elevated temperature rather than of the merely phenolic resin. Besides, after carburization the formed fiber-like nano-powders were good dispersed in PR-BF composite.