Viscosity change rules of Ga-Sb melts with different composition were studied by a viscometer. The results show that the viscosity of Ga-Sb melts increases generally with the temperature decreasing, and the change curves have the feature of exponential change. It is distinct that there is abnormal discontinuous change at 830 ℃ for Ga36.5Sb63.5 melt, which divides the viscosity-temperature curve into two segments, i.e. low temperature segment and high temperature segment. According to the microstructure feature of the melts, the experimental results were discussed.