Based on index ellipsoid equation and characteristics of optical wave propagation and polarization in a biaxial crystal, methods of the collinear phase matching (PM) permitted for frequency doubling in principal planes of biaxial crystals are discussed and analysed. The general expressions of the effective nonlinear coefficients d eff and phase mismatchs for all the allowed PMs are derived for all kinds of biaxial crystals. By using the Table, in which possible PM types or polarization combinations, respective PM angle formula and effective nonlinear coeficients are involved, realizable PM methods may be found and respective PM angles can be calculated. Finally effective nonlinear coeficients for allowed PMs can be compared and optimal PM types or directions can be selected easily.