Isothermal constant strain rate compression tests were carried out for Ti-6Al-2Zr-1Mo-1V alloy. The plastic flow instability was predicted by Prasad criterion and phenomenological criterion and the results were compared through the compression experiment data. It is shown that the both instability criterions predict a large region of flow instability in the high strain rate range of 0.32-10 s-1, which is likely to extend to low strain rate region with decreasing of the deformation temperature. The corresponding microstructures of deformed specimens were observed. The results indicate Prasad criterion can not predict the flow localization and kinking instability in the range of 750-800 oC and 0.001-0.0032 s-1. However, cracks and cavities at initial β boundaries and flow localization can not be predicted by phenomenological instability criterion in the range of 770-870 oC and 0.01-0.32 s-1, and in the range of 900-950 oC and 0.32-3.16 s-1. A new method to predict plastic flow instability was developed by superimposing the two instability maps of the two criterions on the basis of their merits and drawbacks.