The influences of steaming or washing treatment on P-modified HZSM-5 catalyst were investigated systematically in order for getting deep insight into the role of P species in catalytic cracking. The impregnation of P leads to better hydrothermal stability and higher yields of light olefin (ethylene and propylene), but brings rapid decrease of acidity. During steaming, the impregnated P species become more condensed phase and the acidity and catalytic activity decreased further. However, the acidity and catalytic cracking activity recovered just by washing with water even for steamed samples, suggesting that the existence of condensed P species blocks the pores of zeolite reversibly and are responsible for the decrease of cracking activity of naphtha. (C) 2010 Elsevier B.V. All rights reserved.