The calculation of SHG coefficients for crystals with zinc blende and wurtzite structures (AB-type crystals) has been carried out by using the equivalent orbital method. Upon neglect of the electron-phonon interactions, the possibility of adopting the energy bands functions for calculation of these coefficients has been considered. The equivalent orbital method, originally an approximate one in the calculation of the band structures of AB-type crystals, is then used to calculate SHG coefficients for those crystals at K = 0. With some modifications of the band width, the SHG coefficients at K = 0 can be represented approximately as the average of the SHG coefficients for various K vectors. Multiplied by the total number of K vactors in the first Brillouin zone, this gives the macroscopic SHG coefficients for these crystals.Attempts have been made to calculate SHG coefficients for 17 species of crystals with zinc blende and wurtzite structures systematically. It is interesting to note that the values thus obtained agree satisfactorily with the experimental data. Hence the following conclusions can be drawn:1. The one-gap model has been found to be appropriate to zinc blende structures, but it is not the case for wurtzite. 2. It is shown that the coefficients x533(2ω) for the wurtzite structure are the sum of two terms, i.e., contributions from the single state (Γ1,Γ3) as well as from the doublet state (Γ5,Γ6). The former is found to be negative, whereas the latter is positive.3. It is appropriate to use Pauling ionicity as a measure of the ionicity of the A-B bond.