Effects of vanadium on light olefins selectivity of FCC catalysts were investigated with vanadium having different oxidation numbers(hereinafter abbreviated as Oxnum).Molecular modeling studies showed that vanadium with low Oxnum could affect the chemical conversion of large-size hydrocarbon molecules.However,the vanadium deposited on equilibrium catalyst had high Oxnum because of the oxidation reaction taking place in the regenerator,so an activation method to reduce vanadium Oxnum named “selective activation” was introduced.It was proved by means of Electron Paramagnetic Resonance(EPR) and Temperature-Programmed Reduction(TPR) methods that the vanadium Oxnum was decreased,when the catalyst was activated.The molecular modeling studies are consistent well with the lab evaluation results.The light olefins selectivity of activated equilibrium catalysts was better than that achieved by the inactivated catalysts.Similar results were observed with the lab vanadium-contaminated catalyst.The light olefins selectivity of the catalyst was optimized when the vanadium Oxnum was close to 2(VO). Effects of vanadium on light olefins selectivity of FCC catalysts were investigated with vanadium having different oxidation numbers (abbreviated as Oxnum). Molecular modeling studies showed that vanadium with low Oxnum could affect the chemical conversion of large-size hydrocarbon molecules. Host, the vanadium deposited on equilibrium catalyst had high Oxnum because of the oxidation reaction taking place in the regenerator, so an activation method to reduce vanadium Oxnum named “selective activation ” was introduced. It has proved by means of Electron Paramagnetic Resonance (EPR) and Temperature -Programmed Reduction (TPR) methods that the vanadium Oxnum was decreased, when the catalyst was activated. Molecular modeling studies are consistent well with the lab evaluation results. Light olefins selectivity of activated equilibrium catalysts was better than that achieved by the inactivated catalysts Similar results were observed with the lab vanadium-contaminated catalyst.Th e light olefins selectivity of the catalyst was optimized when the vanadium oxide was close to 2 (VO).