The near-field vector electromagnetic distribution in illumination-mode scanning near-field optical microscopes (SNOM) was studied numerically using a two-dimension program based on the boundary element method,taking into consideration of both the sample properties and the polarization of the input light. The numeric results indicated,that in the case of topography-free samples,the power transmission of the optical probe was increased with the increase of the permittivity and the loss angle of the sample material,and that the spot size was hardly effected. However,in the case of topographic samples,the resolution of the SNOM was increased with the increasing of the permittivity and the loss angle of the sample material. Comparison of two operating modes of SNOM indicated that the constantdistance image had a higher resolution for the local change of the sample topography than the constantheight image.