We have grown clean N-salicylideneaniline (SA) single crystals large enough in size by physical vapour depositaion. The photochromism of the SA single is studied by probe light at different optical polarizations At different temperatures and different polarizations,we measured the time dependence of the change of optical density in the colored crystal in the forward and backward reactions.The activation energies of the single crystal are obtained in different optical polarizations.Based on theoretical calculations,the structure configurations of the SA molecules in the colored keto form are analyzed.When an ultraviolet (UV) light is irradiated on the sample at 200 K,about 60% of the ketone form molecules are found to be in the trans-configuration and about 40% of them in the cis-configuration.