HZSM-5 was treated with ammonia water vapor at 373 and 473 K, and the catalytic activities for alkane cracking were evaluated. The treatment removed a fraction of silicon from the framework, but the mesopore was not generated and the micro-pore volume was decreased. The solid acidity was characterized by infrared/mass spectroscopy-temperature programmed desorption (IRMS-TPD) of ammonia experiment. Two kinds of Bronsted acid sites were observed on the unmodified HZSM-5, and the solid acidity did not change largely by the treatment. Catalytic cracking of hexane and octane was enhanced by the treatment, and the second order reaction of cracking became accompanied with the first order reaction. From the products distribution in the propane cracking, the proceeding of bimolecular reaction was confirmed. Discussion was made on the morphology change of the modified HZSM-5 and the resulting enhanced cracking activity. (C) 2012 Elsevier B. V. All rights reserved.