The performance of Ir/gamma -Al2O3 catalyst for the decomposition of high concentration hydrogen peroxide was investigated in a monopropellant thruster. The changes of ignition delay (t(0)), chamber pressure (P-c) and catalyst bed temperature (T-c) with the numbers of startup-shutdown cycles were proved to be effective indicators of catalyst bed efficiency. The fresh catalyst and the deactivated catalyst were characterized with H-2-TPR, XRD and XPS. It was found that catalyst oxidation and surface Sn poisoning are the major reasons of catalyst deactivation.