The present work studied the effects of NaOH concentration on the crystal structure and the reaction rate of zeolite synthesized from a mixture of fly ash and silica powder with a hydrothermal treatment method. As a result, it is found that lower NaOH concentration favors the formation of phillipsite in the generated zeolite by reducing the formation of hydroxysodalite and that the reaction rates monotonously increase with NaOH concentration. It is shown that the crystal structure of generated zeolite depends on factors other than the dissolution rates of silicate ion and aluminate ion from fly ash into NaOH aqueous solution. The fraction of silica powder required for the selective formation of phillipsite increases with the NaOH concentration, while the treatment time decreases with it.