The aim of this study is to investigate the effects of ZnO addition on structural properties of 45S5 glass. Theglasses with nominal composition 45SiO₂-(24.5-x) CaO-24.5 Na₂O-6 P₂O₅-x ZnO (x= 0-15 wt.%) are prepared by meltquench technique and characterized for thermo-physical properties using DTA and micro-hardness measurements. Glass transition and peak crystallization temperature decrease with an increase of ZnO content, which implies the weakening of glass network and increased tendency of glasses towards crystallization.