We investigate the effect of fly ash particle size on the synthesis of zeolites from coal fly ash, and propose and formulate the formation mechanism of zeolite. It is found that the treatment time for the zeolites to synthesize decreases with decreasing feed fly ash particle size. When the feed size is small, both phillipsite and hydroxysodalite are synthesized by hydrothermal treatment with NaOH solution. Only phillipsite is synthesized when large fly ash particles are used. The zeolite from the fly ash having 2.1 mu m mass median diameter indicates the maximum NH4+-adsorption capacity. The change in liquid ion concentrations calculated by the newly proposed model agrees with the experimental results. It is assumed that dissoluble silica is contained by about twice the mass of dissoluble alumina in the large fly ash particles, while in the case of small ones, they are almost equal.