The molecular orbitals of (NO2)-1 grouping were calculated by using the all-valence electron semiempirical CNDO/S method. A general computer program for grouping theory of non-linear optical (ffects was then used to calculate the SHG coefficients of NaNO2. The agreement between the calculated values and the experimental ones was. found to be quite satisfactory without introducing any adjustable parameters. The results show clearly that the anisotropy of SHG coefficients of NaNO2 crystals is primarily due to the π molecular orbitals of (NO2)-1 grouping. The two-level model of SHG effects as well as its limitations have also been discussed in this paper.