The optical model of biological tissue is discussed, in particular its description of optical scattering property. It is found that an equivalent particle size distribution function n(r) can be used to obtain more details and accurate understanding about optical scattering property of tissue. Under the Fraunhofer diffraction approximation, both the scattering coefficient μs and the scattering phase function S(θ) are educible from n(r). The scheme provides a new feasible method based on the light scattering techniques for particle size analysis to measure the optical properties of tissue.