The steady-state expression of local diffuse reflectance from surface of biological tissue is deduced from diffusion theory. Computation result with two set of optical properties of typical colonic tissue is given and compared with the results of Monte Carlo simulation. It is shown that diffusion theory is a conditional approximation. By using a modified expression originated from diffusion theory, the diffuse reflectance is successfully fit with sound enough accuracy, which makes it applicable in deducing optical properties of biological tissues from diffuse reflectance measurements.