Quasiphase-matched second harmonic generation (SHG) has been realized in periodically domain-inverted strontium barium niobate(Sr0.6Ba0.4Nb2O6,so-called SBN) crystal plate induced by an external electric field. Both the coercive field and Curie temperature have been determined throughin situobservation of the second harmonic signals. The measured SHG conversion efficiency is about 10&percent; at a fundamental wavelength of 977.8 nm. ©1997 American Institute of Physics.