Profound and comprehensive knowledge on hot deformation of metastable β titanium alloy is essential to process optimization and microstructure control. In this work, the hot compression behavior of a β quenched metastable β titanium alloy is thoroughly studied by examining microstructural evolution to resolve the related deformation mechanisms. It is demonstrated that the deformation of the alloy presents three characteristic stress-strain stages: a linear stage (Stage I), a discontinuous yielding (Stage II) and a steady-state (Stage III). Before and during Stage I, the β to α phase transformation happened intensively along the β grain boundaries with more than 90% of the boundaries occupied by grain boundaries α (αGB). The linear behavior deviates from the Young's modulus when the linear deformation proceeds to the late stage, indicating the onset of plastic deformation. This behavior results from dislocation slip near β grain boundary regions. The discontinuous yielding of Stage II is originated from the fragmentation of the αGB and the intensive formation of mobile dislocations near the β grain boundary areas. The steady-state of Stage III arises from two orientation dependent deformation modes of the β grains. For those in favorable activation orientation for their {110}β<11>β systems with respect to the external compressive load (type I), the deformation is realized by the dislocation slip and by the formation of slip bands; whereas for those with their {1}β<1>β systems - the major lattice deformation for structure change from β to α - in favorable activation orientations (type II), the deformation is achieved by intensive phase transformation by forming intragranular α precipitates. The present results provide new information on hot deformation mechanisms of metastable β titanium alloys, especially the phase transformation associated lattice deformation as an important plastic deformation mode.