The phase matching for second harmonic generation(SHG) in biaxial nonlinear-optical crystals is discussed and the accurate calculation of the corresponding effective nonlinear coefficients is presented, based on which the optimum phase matching can be determined. As an example, numerical treatment of phase matching and the corresponding effective nonlinear coefficients for SHG at fundamental wavelengths of 1.064μm and 0.808μm in the organic nonlinear-optical crystal benzophenone is performed, and accordingly the optimum phase matchings are found.