This paper discusses the calculation methods of the PM loci and the effective second-order nonlinear coefficients dIeff and dIIeff of three wave interactions, such as SF, DF, OPO and etc. in biaxial crystals. It is found that the optimum PM angles ?OPM and ?OPM can be calculated by computers. The paper also reports the computed results for 6 biaxial crystals and the experimental results for 2 biaxial crystals. The computed resulte agree quite favorably with the experimental results.