This article discribes a comparative method for experimental determination of SHG coefficients of some crystals. With the d36KDP as a reference standard, we measured both d36KDP and d31BNN, d32BNN of several samples of BNN crystal. Their results show that the coefficient d32RNN is sensitively related to the stoichiometric composition of BNN crystal. Some discussions concerning the nonlinear optical properties of BNN crystal are also presented.