The objective of this study is to investigate the cation exchange property of the product (zeolite P) synthesized from coal fly ash by alkali hydrothermal reaction, and to evaluate the water purification ability for simultaneous removal of NH4+ and PO43- in aqueous solution. Zeolite P crystal begins to be collapsed or to be dissolved with a decrease in aqueous pH. The cation exchange capacity of the product drastically decreases below pH 4. The simultaneous removal of NH4+ and PO43- can be achieved when NH4+ is exchanged by CaP zeolite and an insoluble salt is formed by the reaction between Ca2+ released from zeolite and PO43- in aqueous solution. From the above results, the effect of pH on cation exchange property of the product and the mechanism of simultaneous removal of NH4+ and PO43- in aqueous solutions are considered, respectively. The CaP zeolite used in this study, which can simultaneously remove NH4+ and PO43- has advantages compared with the other removal methods.