The origin of nonlinearity inKTiOPO4was investigated quantitatively from the chemical bond viewpoint. All constituent chemical bonds in this crystal were considered and their contributions to the total linearity and nonlinearity were quantitatively determined. Calculated results agree satisfactorily with experimental data in both signs and numerical values. These results show us thatTiO6groups and P(1)O4groups have relatively larger linear contributions and the nonlinearity derives fromKOx (x=8,9)groups and P(2)O4groups. ©1997 American Institute of Physics.