Abstract: A Ce conversion coating, which is non-toxic and non-pollution, was prepared by chemical dipping immersion treatment on graphite fibers reinforee aluminum (Gr/Al) eoraposite surface. It is shown that the graphite fiber and aluminum alloy surfaces both are coated by the Ce conversion coating, and the microstructure of the coating is dried-mud-like. Energy dispersive spectroscopy (EDS) indicates that the major elements distributing on the coating are Al, O, C, Si, Ce, and Mg, and the mass fraction of Ce is 14.44wt% after coating for 30 rain but increases to 47.48wt% after coating for 120 min. The formation meehanism of coating mierocracks was put forward. The micro-cathodic formation mechanism can explain the characters of the coatings. The Ce conversion coating becomes a barrier of O₂ diffusion and electron transmitting, and oxygen absorption and hydrogen separation were both inhibited.