A technique for observation of 180° domains in ferroelectric crystals is demonstrated by the use of the second harmonic generation (SHG) microscope. The phase reversal of the SH wave accompanying inversion of spontaneous polarization is exploited to visualize domains. Interference between SH waves converts the phase information to the SH contrast. The domain mapping is achieved in LiNbO3and LiTaO3by nonlinear coefficientsd33andd22under the microscope, which enables the characterization of a periodically poled structure in quasi-phase-matched wavelength converters in a nondestructive way. The validity of the technique is proved by another characterizing tool of destructive etching. ©1997 American Institute of Physics.