We demonstrate that quasi-phase-matched second harmonic generation occurs in annealed proton-exchanged (APE) X-cut MgO 5 mol % doped LiNbO3 (MgO:LN) waveguides supporting a TE-mode guided wave. Formation of a domain inversion was achieved by applying electric fields in X-cut and Y-cut MgO:LN. The depth of the formed domain inversion was 1 μm with a period of 4.75 μm. Anomalous characteristics of current versus voltage (I–V) of samples having domain inversion formed therein were found to have similar properties to a diode. A blue light having a wavelength of 475 nm and a power of 2.2 mW has been obtained with a normalized second harmonic conversion efficiency of 60%/W cm2 with uniform grating of 6 mm by using a Ti:Al2O3 laser as a fundamental wave source. © 1997 American Institute of Physics.