The nonlinear optical phenomena in a three-dimensional LiNbO3 optical waveguide are discussed. The phase-matching condition and conversion efficiency of second harmonic generation and difference frequency generation are derived theoretically. The temperature dependence of birefringence of the LiNbO3 crystal is used to satisfy the phase-matching condition of nonlinear interactions. It is revealed that the temperature-tuning method is very useful in satisfying the phase-matching condition over a wide wave-length range. The several percent conversion efficiency is expected with 100-mW fundamental or pumping power and a 1-cm-long optical waveguide. The second harmonic generation and difference frequency generation experiments using Ti in-diffused LiNbO3 optical waveguides were also shown. The maximum conversion efficiency for second harmonic generation (SHG) was measured to be 0.77% for 65-mW fundamental input power. Infrared waves ranging from 1.035 to 1.19 μm were generated by the difference frequency generation.