The characteristics of hot deformation of an α+β titanium alloy Ti-6.5Al-3.5Mo-l.5Zr-0.3Si with acicular microstructure were studied using isothermal hot compressive tests in a strain rate range of 0.01-10 s-1 at 860-1 100 ℃. The true stress-true strain curves of alloy hot-compressed in the α+β region exhibit a peak stress followed by continuous flow softening; whereas in the β region, the flow stress attains a steady-state regime. At a strain rate of 10 s-1 and in a wide temperature range, the alloy exhibits plastic flow instability. According to the kinetic rate equation, the apparent activation energies are estimated to be about 633 kJ/mol in the α+β region and 281 kJ/mol in the β region, respectively. The processing maps show a domain of the globularization process of α colony structure and α dynamic recrystallization in the temperature range of 860-960 ℃ with a peak efficiency of about 60%, and a domain of β dynamic recrystallization in the β region with a peak efficiency of 80%.