The surface magnetic and structural properties of magnetite thin films have been probed by nonlinear second-harmonic generation optical method in high- (centrosymmetric, Oh) and low- (noncentrosymmetric, C1) temperature phases. A model taking into account the cubic magnetocrystalline anisotropy and magnetic symmetry reduction due to a magnetically modified surface layer is supposed to describe the azimuth variations of the nonlinear response. The metal-isolator transition (Verwey) manifests itself in an increase of nonlinear response (~25%) and a decrease in magnetic contrast (~50%).