V2O5/MgF2 catalysts with V2O5 contents ranging from 2.1 to 15.7 wt% were prepared, and the influence of the V2O5 content of the V2O5/MgF2 catalyst on the structure and activity for the ammoxidation of 3-picoline was investigated. XRD data indicate that V2O5 is in a highly dispersed state through segregation of V2O5 into tiny crystallites occurs at and above 8 wt% V2O5. The 3-picoline ammoxidation activity increased with an increase in V2O5 content due not only to the species arising out of interaction of V2O5 and MgF2, but also to the presence of V2O5 microcrystals in the catalysts.