The Representative Volume Element (RVE) approach by homogenization theory was used to investigate the stiffness of cancellous bones. Two-level micro-structural model for canceilous bones was proposed, in which the trabeculae and cancellous bone are separately simulated in order to consider the influence of the structure of trabeculae. It is shown that the relation between the elasticity and the apparent density for cancellous bone is nearly linear. Results obstained from two-level model are higher than those from the isotropic model.