Carbon fibers had been modified by the method of electroless plating Ni. The surface morphology, ingredient and cross-section images of modification carbon fibers were charactered by scanning electron microscope and X-ray diffraction. A continuous dense Ni-P alloy layer could be coated on the surface of carbon fibers by such method. The nickel-plated layer expanded in a layer growth pattern. The reaction thermodynamic analysis indicated that the process was spontaneous. The controlling process of electroless plating Ni on the carbon fibers could be divided into 3 stages: at first, controlled by chemical reaction; hereafter, controlled by chemical reaction and ions diffusion; finally, controlled by ions diffusion.