Ammoxidation of methyl aromatics to their corresponding nitriles over Silica-Supported vanadium phosphorus oxide (VPO/SiO2) catalysts has been Studied. The effects of carrier silica, the addition of Phosphorus, the Substituents and the loadings oil Catalytic performances have been discussed. Compared with unsupported VPO, the VPO/SiO2 catalysts have higher,hcr catalytic activity for ammoxidation of Substituted toluenes at much lower reaction temperature. XPS shows that vanadium and phosphorus exist mainly in the pentavalcmstate in the VPO/SiO2 catalysts. XRD shows that vanadium phosphorus oxides exist as amorphous phase in 10 wt.% catalyst. When the loadings, are over 10 wt.%, the crystalline alpha-VOPO2, Would emerge. which would decrease the yield and selectivity. Additional phosphorus can inhibit the formation of crystalline V2O5 and form amorphous composite oxides with vanadia, which play concerted catalytic function and increase the Selectivity of nitriles remarkably. The methyl aromatics with different Substituents or same Substituents on diffetent positions have different catalytic activities because of the Variant electronic stability of intermediates, the hindered accessibility of methyl group or the chemisorption state of the Substrate molecule On the electrophilic catalyst Surface.