The effect of different length-diameter ratios of carbon fibers (CF) on damping property of CF/EP composites was investigated. The CF was coated with copper structurally and the copper coated CF (Cu-CF) was verified by SEM. The mechanical,electrical and damping properties of composites were studied. The results show that the copper plating is coated on the surface of CF uniformly; the addition of a little large length-diameter ratio CF can enhance the impact strength of CF/EP composites effectively, while the bending strength decreases; the addition of more small length-diameter ratio CF can also improve the mechanical properties of the composites. In addition, the large length-diameter ratio CF can heighten the conductivity and damping property of composites easily.