Abstract: In this paper, the difference in photochemistry properties between nitrosamine and pigments is investigated. The TLC method of twice developing light-decomposition is presented to detect the volatile nitrosamines in grain.First, nitrosamine and pigments are developed within a certain distance and, after that, the nitrosamine is decomposed under irradiation with ultraviolet light. When they are developed for the second time, the pigments keep going up and are being completely separated from nitrosamine.