Seventeen years ago, Ito, Naito, and Inaba [J. Appl. Phys.46, 3992 (1975)] studied the general nature of induced second‐order nonlinear optical polarizations and their phase matching in biaxial crystal. They derived the expressions of the effective nonlinear coefficients for both type I and type II phase matching. In recent research work, we found some errors in some of the expressions in that paper which were used by some papers published later.