The crystallization kinetics of the two commercial bioactive glasses, 45S5 and S53P4, was studied using differential thermal analysis (DTA), optical microscopy, and scanning electron microscopy (SEM). The thermal properties, the activation energy of crystallization, and the Johnson-Mehl-Avrami (JMA) exponent were determined for two glass fractions: fine powder (<45 μm) and coarse powder (300–500 μm). The crystallization behavior of 45S5 was significantly different for the two fractions, whereas the particle size did not affect the crystallization behavior of S53P4. The JMA exponent of S53P4 suggested surface crystallization for both size fractions. However, for 45S5, the JMA exponent suggested that, with increasing particle size, crystallization evolves from predominantly surface crystallization to predominantly bulk crystallization. Surprisingly, SEM imaging did not support this conclusion. A method based on the crystallization rate dα/dt showed that the JMA approach could not be employed for 45S5. The crystallization mechanism of 45S5 appears to be more complex than a simple nucleation and growth process. Nucleation-like curves were measured for both fractions of the two glasses. The maximum nucleation rate occurred at 566 ± 4°C and 608 ± 4°C for the coarse powders of 45S5 and S53P4, respectively. The higher maximum nucleation temperature of S53P4 was attributed to the higher SiO2 content. The nucleation temperature range of these two glasses together with DTA data makes it possible to develop guidelines for tailoring thermal treatment parameters to achieve desired glass-to-crystal ratios.