We demonstrate that quasi-phase-matched second harmonic generation occurs in annealed proton-exchanged (APE)X-cut MgO 5 mol &percent; dopedLiNbO3(MgO:LN) waveguides supporting a TE-mode guided wave. Formation of a domain inversion was achieved by applying electric fields inX-cut andY-cut MgO:LN. The depth of the formed domain inversion was 1 &mgr;m with a period of 4.75 &mgr;m. 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 of60&percent;/W cm2with uniform grating of 6 mm by using aTi:Al2O3laser as a fundamental wave source. ©1997 American Institute of Physics.