Natural zeolites were modified with 1-5N of mineral acid(H₂S0₄, HCl,H₃P0₄) and ammonium salt((NH₄)₂S0₄,NH₄Cl,NH₄N0₃,N H₄F) aqueous solutions followed by calcination at 500⁰C and their catalyticactivities for various acid-catalyzed reaction such as dehydration of 2-propanol,cracking of cumene and nhexane,disproportionation and alkylation of toluene,conversion of methanol,etc. were investigated by use of fixed bed flow reactor. It was revealed that the catalytic activities of natural zeolites for these acidcatalyzed reactions were remarkably enhanced by the modification with these acid and ammonium salt solutions. The cause of enhancement of catalytic activities of natural zeolites by modification with these acid and ammonium salt aqueous solutions was supposed to be due to the increase of surface acidity of natural zeolites by these pretreatmentsIt can be concluded that there is a possibility of usage of modified natural zeolites as highly active catalysts for various hydrocarbon conversions instead of synthetic zeolites.