We investigated the effect of fly ash content on the synthesis of zeolites front coal fly ash prepared by hydrothermal treatment. Both phillipsite and hydroxysodalite are synthesized from fly ash of relatively low silica content. It is found that only phillipsite is synthesized selectively by hydrothermal treatment when concentration of silicate ion is higher than that of aluminate ion in NaOH solution. We proposed and formulated the formation mechanism of phillipsite, Silicate and aluminate ions dissolve in NaOH solution from fly ash and form aluminosilicate, from which phillipsite crystal are generated. The changes in ion concentrations and phillipsite mass calculated by the newly proposed model agrees with the experimental results. It is suggested that the critical concentration of crystallization and the rate constants of gelation and crystallization are constant, independent of fly ash morphology.