Theoretical descriptions of the molecular nonlinear optical properties of tetrahedral donor-acceptor molecules are presented by using a valence-bond and four charge-transfer state model. Based on this five-state model, as the extent of the charge transfer from the peripheral donors (acceptors) to the central acceptor (donor) increases, the first hyperpolarizability monotonically increases. The theoretical predictions are confirmed by carrying out ab initio calculations of the first hyperpolarizabilities of three different series of tetrahedral molecules. The pi-electron delocalization effect on the nonlinear optical property is elucidated by making a comparison of the first hyperpolarizability of the tetrahedral molecule with that of fictitious tetrahedrally assembled linear polyynes.