We have grown clean N-salicylideneaniline (SA) single crystals large enough for optical measurements. The photochromism of a SA single crystal is investigated at 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 40% of the keto form molecules are found to be in the cis configuration and about 60% of them in the trans configuration. The activation energies of the single crystal are obtained in different optical polarizations, and the energy-level schemes of the photochromic reactions are established. Investigation of a single crystal can provide us with new information on the photochromism of SA, because SA molecules are regularly arranged and the environment of each molecule is the same.