Nano-silica were grafted onto the surface of CF by the cross reaction of silane-coupling. In order to investigate the best modification condition, the effects of the content of nano-silica, reaction time and temperature were studied. The surface morphology of the modified samples during the preparation were characterized and the structures of them were analyzed by FTIR. The results showed that the best modification conditions obtained were as follows: the SiO₂content was 4 % , reaction temperature was 100 ℃ and reaction time was 4 h. Under optimum conditions, the grafting rate of Nano-SiO₂was 5 . 69 % .