Abstract: Vanadium phosphate oxide (VPO) catalyst precursor was activated under the atmosphere of air and steam/air (S/A) mixed gas respectively. The activated catalyst samples were characterized by XRD, XPS and SEM to study their crystal structure and surface property. Results showed that after activation under S/A mixed gas and air, the main crystal phase of the catalyst was (VO)2P2O7 and VOPO4, respectively. The presence of water vapor during activation reduced the average valence state of vanadium (AV)，but increased the P/V atomic ratio of catalyst surface. The catalytic performance of these prepared catalysts was evaluated by selective oxidation of n-butane to maleic anhydride reaction. It was shown that catalyst FVO-1 activated by mixed gas with S/A volume ratio of 1/3, having an AV of 4.198 and P/V atomic ratio of 1.17 exhibited good catalytic performance: its conversion of butane was 87.10% and the selectivity of maleic anhydride reached 72.89%.