In this paper, the variation of the phase matching angle and the effective nonlinear coefficient versus the fundamental wavelength in a second harmonic generation of the lithium formate roonohydrate (LiCOOH ‘HaO)of the mm 2 type crystal are calculated. It is found that there is one (or two) optimum phase matching angle {θ, φ) for each fundamental wave, at which the second harmonic conversion efficiency is highest. Fitted formula for the variation of the phase matching angle versus the fundamental wavelength is obtained.Second harmonic generation for 0.53μ, 0.35 μ and 0.266 μ are observed from the crystals cuied at the computative angle.