The number density of photons absorbed in semi-infinite plane, simple columnar and double columnar biological tissues is numerically studied with Monte-Carlo method, respectively. The same optical parameters are used in the simulation for the three situations. The results show that the boundary employed in the simulation has important effect on the light propagation in tissue. The effect of boundary on light propagation in tissue is investigated. Energy flow rate in biological tissue as a function of radial distance is simulated and the results agree well with experimental results.