In order to accurately identify the size and material of contaminants about the wafers and optical elements, the scattering model about two kinds of slightly non-spherical particles on wafer is established by the Bobbert and Vileger (BV) theorem. The scattering process is analyed and the scattering coefficients are derived through expansion vector spherical harmonic function. The figure about the differential scattering cross setion (DSCS) of a non-spherical particle is calculated, which is compared with that from extended Mie. The results are coincidence, which proves the validity of the method. The effect of different material non-spherical particle deformations is reviewed by the calculation. The result shows that the effect of the dielectric is smaller than that of the metal. Therefore, the material of the defect and the shape can be extracted by calculating the DSCS.