The subsolidus phase relations of the ternary system ZnO–B 2 O 3 –V 2 O 5 were investigated by means of X-ray diffraction (XRD). There is no ternary compound in the system. There are seven binary compounds which have been reported in the ZnO–B 2 O 3 –V 2 O 5 system, which can be divided into eight 3-phase regions. The phase diagram of Zn 4 V 2 O 9 –Zn 3 B 2 O 6 pseudo-binary system from 740°C to 1250°C has also been constructed through XRD and differential thermal analysis (DTA) methods, and the result reveals this system is eutectic system. The eutectic temperature is 780°C and eutectic point comprises 55% Zn 4 V 2 O 9 and 45% Zn 3 B 2 O 6 . The mixture with the composition near the eutectic point might be the suitable flux for ZnO crystal growth.