The crystallization kinetics of fly ash based glasses were investigated by the Johnson-Mehl-Avrami (JMA) equation using Differential Thermal Analysis (DTA) data. In DTA experiments, glasses having different particle sizes were used and the results were compared. It was found that the value of the Avrami exponent was about 2.5 for the coarse sample, whereas it was near 1.08 for the fine sample. Activation energies were determined for coarse and fine glass samples as 297 and 348 kJ/mol, respectively. Cross-sectional SEM investigations clearly demonstrated that the bulk crystallization is the predominant mechanism in the glass-ceramic sample.