Abstract: Diffusion approximation (DA) theory is often used as basis of the experiments and methods in determining the optical properties of biological tissues. However, DA is only an approximation of the accurate radiative transfer theory and has its validity range under given conditions.In order to assess the validity of the DA under the requirement of in vivo non-invasive measurement, we compared the results from the DA theory with the more accurate results from the Monte Carlo methods.A numerical validity criterion for the DA is established on the basis of the single-scattering albedo a and anisotropy factor g for semi-infinite tissus with index-matched boundaries.