Plenty of nonlinear optical (NLO) crystals containing VO4 and AsO4 tetrahedra as NLO-active units have been found. Among them, Li3MO4 (M = V, As) and LiA(2)MO(4) (A = Rb, Cs; M = V, As) are isostructural, respectively; however, their second-harmonic generation (SHG) response is vastly different, and the mechanisms of their SHG responses are still unclear. Through the band-resolved and SHG-density method, it shows that the MO4 (M = V, As) tetrahedra give the dominant contributions to the SHG response since the stable covalent interactions are within MO4. The reason for the disparity in SHG response is the strong d-p hybridization in vanadates as well as the weak s-p hybridization in arsenates. This conclusion not only demonstrates that alkali metal vanadates are potential NLO materials for NLO application and the mechanisms of the VO4 and AsO4 groups tetrahedra are different but also identifies that these tetrahedral groups might be interesting and useful NLO-active units for synthesizing new NLO materials.