We have found some inappropriate points in calculating the effective NLO coefficients (deff) of biaxial crystals described in literature. The correct calculation method is developed. As an example the calculation method for biaxial crystals with symmetry of mm2 point group is presented in detail. These formulas have been borne out by experimental results. The calculation method makes it possible to design optimum phase matching for all kinds of biaxial crystals with a microcomputer.