We use the Monte-Carlo method to calculate the intensities of transmission and reflection of a collimated light beam scattered from randomly distributed particles in thin layer. The results obtained are compared with those calculated by the transport theory of diffusion approximation. When the scattering is nearly isotropic, both of the results agree well. When scattering becomes anisotropio, the results from Monte-Carlo method are reasonable, while the theory of isotropio scattering approximation becomes invalid.