The devitrification phenomenon and nucleation of CaO/P 2 O 5 glass with Si 3 N 4 additive was investigated using thermal analysis techniques. To determine whether the devitrification begins at the surface or in the bulk of these glasses, we compared the exothermic peak temperature of DTA curves for coarse and fine particles, and it was found that the devitrification mechanism of CaO/P 2 O 5 glass would change by adding Si 3 N 4 powder. The addition of Si 3 N 4 additive will induce the calcium phosphate glass to surface nucleation mechanism in this work, and the temperature of exothermic peak of DTA curves for calcium phosphate glass were also shift to higher temperature by increasing the weight percentage of Si 3 N 4 . On the other hand, the nucleation rate of calcium phosphate was decreased with increasing the weight percentage of Si 3 N 4 .