The sewage treatment sludge disposal has become a serious environmental problem because of restricted direct landfilling and oceandumping in spite of their large amounts discharged. So the recycling of sewage treatment sludge is very useful alternative for waste management. Here, we studied the synthesis of zeolite in open system from the sewage treatment sludge incinerator fly ash by means of hydrothermal synthesis. We considered the concentration of NaOH, the reaction time and the reaction step as synthesis variables. The phase of zeolite products was identified by X-ray diffraction(XRD) analysis and ammonium ion exchange test was performed for the raw fly ash and two zeolite products(Z-3 and Z-5). In leaching test of the raw fly ash, hazard metal is detected very low level compared with regulatory leaching test standard. But in total recoverable test, the total contents of the fly ash were very high in terms of the standard for waste-derived fertilizer. Through hydrothermal reaction, small amount of zeolite P was synthesied in 1 N of NaOH solution and relatively large amount of hydroxysodalite was synthesied in 3 N and 5 N of NaOH solution with similar peak intensity. Addition of an aging step in the synthesis didn t increase the amount of zeolite phase. Maximum NH4+-N exchange capacity is 1.49 mg NH4+-N/g in Z-3 and 1.38 mg NH4+-N/g in Z-5. Most of the ammonium ion is exchanged in 30 minutes and disorption did not occur until 5 hours.