Luminescence from porous silicon is studied with plane polarized light excitation under grazing incidence. The experiments show that the incident angle of light almost has no influence on the luminescence behavior of porous silicon. However, the luminescence with z polarization excitation is found to be higher in intensity than that with x polarization excitation. The different orientations of the electric field of the exciting light with respect to the sample surface result in the difference in the excited luminescence, which reveals the anisotropy of the optical properties of porous silicon and rules out the assembly of pure quantum silicon dots as the structure of porous silicon.