The thiohydantoin derivative derived from amino acid was used for the stepwise sequence analysis of peptide or protein from the carboxyl termini. Recently, SUZUKI reported the mass spectrometric identification about a part of these compounds. In this paper, was described the mass spectrometric identification of thiohydantoins derived from 20 amino acids. Mass spectra were obtained with a mass spectrometer, JEOL model JMS-06H and samples were introduced with a direct inlet probe. The molecular ion peaks and fragment ion peaks were identified easily, because these peaks appeared differently every amino acids and specially, it was easy discrimination between leucine and isoleucine. It is suggested that mass spectrometry was one of the useful methods to identify thiohydantoins derived from amino acids.