The effect of silica content of fly ash on the synthesis of zeolites from fly ash by hydrothermal treatment was investigated. Both phillipsite and hydroxysodalite are synthesized from fly ash of low silica content. Only phillipsite can be obtained from a mixture of low silica fly ash with silica powder whose dissolution rate constant is relatively large. Phillipsite can be also synthesized from fly ash of low silica content selectively, using additive silica solution or waste NaOH aqueous solution with which fly ash of high silica content has been treated hydrothermally. It was clarified that the crystal structure of synthesized zeolite depends on the ratio of silicate ion to aluminate ion in NaOH aqueous solution. The crystal structure and absorption capacity of synthesized zeolite can be estimated from the ratio of silicate ion to aluminate ion at 90 minutes calculated from the experimental conditions.