The inherent external mismatching angle is used to represent the phase mismatching caused by inhomogeneity in refractive-indices of doubling crystals. A new method is presented that can measure this important parameter accurately. The small-signal conversion efficiency is measured under a laser intensity of about 20 MW/cm2. A reference KDP crystal is used to eliminate laser-induced fluctuations. The experimental data are compared with theory in determining the mismatching angle of crystals, Δθc, as listed in Tables II，III, Among doubling crystals those with high conversion efficiencies can thus be selected readily. For some of then L, the high conversion efficiencies have been born out experimentally.