Using acousto-optic Q-switched Nd3+: YAG laser as a source of fundamental beam, we measured the SHG scan vs temperature in LiNbO3 and BNN single crystals. There are usually two peaks appearing on the SHG scan curves for the striae region, at the same time two NPM hollow cones of SHG light may be observed in one to one correspondence.One of the two peaks in the polydomain striae region of LiNbO3 disappeared after the crystal was completely polarized. The occurrence of this peak was analyzed.The SHG power in the polydomain region of LiNbO3 is usually several times larger than that of single domain region. We attributed the enhancement mechanism to the stress relaxation occurring with the formation of insular 180°domains.