Abstract: The effects of orthophosphoric acid concentration in precursor preparation with n-pentanol as reductive agent on vanadium catalyst behavior were studied. For the catalysts prepared at higher concentrations of the acid, the quantity of V⁴⁺ phase (VO)₂P₂O₇ is very large,and conversions of n-butane to maleic anhydride(MA)are all comparatively high. With decreasing acid concentration in precursor preparation, the relative amount of V⁵⁺ phase δ-VOPO₄ and n-butane conversion increase continuously, whereas the selectivity to MA increases at the beginning and then decreases when it reaches a maximum value. The catalyst prepared at the optimal acid concentration of 101.7% gave the maximum yield of MA of 47.67 %(mol) at an inlet n-butane concentration of 4.0％ and space velocity of 500 h^-1.