The random anti-parallel micro-ferroelectric domains in SBN (Strontium-Barium Niobate, Sr0.61Ba0.39Nb2O3, ) crystal can be used for efficient nonlinear optical processes at different frequencies. The ferroelectric domains in SBN crystal are easily influenced by temperature and electric field. By comparing different second harmonic generation (SHG) profiles and efficiencies of SBN crystal under electric field poling and thermal treatments at different temperatures, and observing the ferroelectric domain structures (sizes and directions) under the scanning electronic microscope (SEM), the mechanism of how the thermal treatment and poling affects the SBN crystal’s inner structure, thus the frequency doubling process, is revealed. We found that for the poled SBN crystal, the SHG emission shows a speckle pattern; on the other hand, after a thermal treatment there is a clear maximum in the propagation direction of the fundamental light.