Isothermal constant strain rate compression tests of TA15 titanium alloy with basket-weave and Widmannstatten two types of beta transformed microstructurehave been carried out in the temperature range of 750℃-950℃ and strain rate of 0.001s_-1-10s_-1. The results show that there is a wide range of plastic flow instabilitySof the two beta transformed microstructure at low temperature and high strain rate region (750℃-880℃,0.0032s_-1-10s_-1).The instability range of Widmannstatten microstructure is larger than that of basket-weave. In the range of 750℃-775℃,0.001s_-1-0.0032s_-1 and 880℃-950℃,0.56s_-1-10s_-1 the Widmannstatten microstructure also have plastic flow instability defects.The instability defects of Widmannstatten microstructure have 45° macro shear crack, flow localization band and microcrack. The basket-weave microstructure has 45° macro shear crack and flow localization defects. The plastic flow instability is more likely to happen in Widmannstatten than basket-weave microstructure is primary cause of the difference of the initial alpha lamellar thickness in two beta transformed microstructure.