We prepared a potent and relatively selective human chymase inhibitor 9 (-), based on the study of SAR of a symmetrical anhydride-type serine protease inhibitor 1. Kinetic studies suggested that 9 (-) reacts with the Ser residue at the active site of the enzyme, forming a stable acyl enzyme complex. We also showed the importance of the tri-substituted beta-amino acid structure for the potent anti-enzymatic activity.