The effect of B2O3 addition on the thermal stability of BaO-P2O5 glasses is studied by differential thermal analysis (DTA), X-ray diffraction (XRD) analysis, scanning electron microscopy, and micro-Raman spectroscopy. The difference between glass-transition and onset-crystallization temperatures increases monotonically with increasing B2O3 concentration. The DTA result reveals that no exothermic peak due to surface crystallization exists in the BaO-P2O5 glass doped with 3 mol% B2O3. A single-mode BaO-P2O5-B2O3 glass fiber could be fabricated by a rod-in-tube technique. The modification of glass structure due to B2O3 addition is qualitatively discussed.