Highly efficient second‐harmonic generation using a titanium in‐diffused LiNbO3three‐dimensional optical waveguide was performed with the cw Nd : YAG laser at 1.064‐&mgr;m wavelength. The phase‐matching condition to convert the TE00fundamental frequency to the TM00or TM20second harmonic was demonstrated by adjusting the crystal temperature. The conversion efficiency is 1.5×10−4at 2‐mW fundamental input power under the phase‐matching condition. By using a low‐loss LiNbO3optical waveguide, equipped with highly reflective cavity mirrors on the input and output surfaces, parametric oscillation is expected.