By the 90 deg. elastic light scattering investigation and far field observation in the range of 20-800 centigrade, the relation between behavior of light scattering anomalies and evolution of nanodomain structures in lattice of barium sodium niobate (Ba2NaNb5O15, BSN) crystal was clarified. The correlation between anomalies on the temperature curves of the elastic light scattering intensity and temperature transformations of nanodomains was studied by X-ray and electron microscope methods. Phase transition near 500 centigrade and movement in field of scattering light could be explained by appearance of a new incommensurate phase.