The authors have proposed a general constitutive theory based on dissipative function, to describe inelastic behavior of heterogeneous materials. In this paper, the authors formulated a constitutive law for granular materials using basic idea developed in the general theory.The dissipative function used in this paper is determined by formulating the frictionalmechanism as well as the trapping of elastic energy in the shearing process of a 3-dimensional granular model with regularly packed spheres. The stress-strain relationship determined by the constitutive theory is compared with the result of GEM simulation test, which verifies the usefulness of our theory. It is proved in this paper that it is possible to deduce dissipative function from micromechanical point of view for the simple model.