Abstract: A review concerning the valence state of vanadium on catalysts during catalytic cracking process, factors affecting the variation of vanadium valence states, poisoning effect on catalysts caused by various vanadium oxides and mechanism of poisoning, as well as research methods, characterization tools and essential results, is presented. It can be concluded that the negative effect on the structure and performance of FCC catalysts by high valence state of vanadium species is much more serious than that of by low valence state vanadium species.