The competitive water adsorption in the practical environment usually causes severe deactivation of the O-3 decomposition catalysts. In this study, MnCO3 was used as precursor to prepare efficient and stable MnO2 catalyst for gaseous O-3 decomposition in humid stream. The sample S-300, prepared by calcining MnCO3 at 300 degrees C for 6 h, exhibited 63% (80%) of removal efficiency for 120 (43) ppm of O-3 under a high space velocity of 600 L g(-1) h(-1) and 50% (40%) of relative humidity at 25 degrees C, which is superior to the commercial O-3 scrubber (oxide compound of Cu and Mn) and alpha-MnO2. However, even lower or higher treatment temperature did not further promote humid O-3 decomposition. Thorough characterizations, especially by the temperature programmed and in situ DRIFTs experiments, demonstrate that the large amount of active oxygen vacancies and strong Lewis-acid sites together with the easy recovery of the occupied oxygen vacancies and weak interaction with water molecules accounted for the excellent O-3 decomposition activity in humid stream. Besides, the developed pores by thermal decomposition of MnCO3 might also facilitate O-3 removal. This study provides an easy and rational design of affordable and efficient catalysts for environmental application.