Perceiving granular material as a collection of particles, a constitutive law for granular material is derived based on micromechanics approach, taking into account the mechanisms of sliding and separation of particles under large deformation. The derived constitutive law is explicitly in terms of inter-particle contact properties. In deriving the overall stress-strain relationship, heterogeneity of the material is considered by a newly introduced distributive tensor. The distributive tensor describes the strain fluctuation in the system and facilitates an averaging process for obtaining the overall deformation behavior. Predicted results from the present model are compared with that obtained from computer simulation and experiments on sands. Unique features of the present theory are discussed to show their usefulness in comprehensive modelling of complex behavior for granular material.