The chemisorption of CH3NO2 on Cu(111) surface has been studied by the semiempirical quantum mechanical method (CNDO/2) of cluster models. The results indicate that the bridge site of Cu(111) is preferable for CH3NO2 adsorption, the orientation of CH3NO2 adsorption is that -NO2 points downward to the Cu surface, and the angle between the plane ONO and Cu-Cu bond is 60o. The orientation of adsorption and the density of states calculated by CNDO/2 calculation are in agreement with the experimental results. The analyses of atomic orbital percentages show that CH3NO2 adsorption is the result of interaction of 2p atomic orbitals of 0 for CH3NO2 with 3d atomic orbitals of Cu.