USY zeolites prepared by steam treatment of NH4-Y zeolites were treated with ammonium nitrate solutions. Treatment with ammonium nitrate solutions resulted in a remarkable enhancement in the octane-cracking activity of the USY zeolites. This enhancement in the octane-cracking activity was attributed to the increase in the amount of acid sites and the enhancement of the acid strength, as observed in the ammonia IRMS-TPD method.