Abstract: Physicochemical and catalytic properties of steam treated phosphorus-modified- HZSM-5 zeolites have been investigated. XRD and IR show that no new crystalline phase exists in the modified samples. Acidic properties of the samples have been characterized by ammonia temperature programmed desorption (NH3-TPD), indicating the introduction of phosphorus by impregnation of the zeolite with an aqueous solution of NH4H2PO4 and steam treatment decrease the number of acid sites. After steam treatment, BET surface area decreases remarkably due to the blockage of channel by phosphorus species and dealuminated species. The nitrogen adsorption-desorption measurements suggest that the isothermal type of all samples is a combination of type I and type IV, all hysteresis loops resemble the H4-type in the IUPAC classification. The conversion of n-heptane over steam treated P-modified-HZSM-5 zeolite is higher than that of sole steam treated zeolites, showing a protective effect of the phosphorus against dealumination by hydrothermal treatment at elevated temperature.