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Hot compression deformation behavior of Ultrafine-grained (UFG) commercially pure (CP) Ti with the average grain size of 200 nm was studied by thermal simulation test at the deformation temperature of 298-673 K and the strain rate of 10-3-100 s-1 on the Gleeble-1500 thermal-mechanical simulator. UFG CP-Ti was produced by ECAP up to 8 passes with a die of Φ=120° using route Bc at room temperature. The microstructural evolution of UFG CP-Ti was investigated by transmission electron microscopy. The results show that the flow stress increases with increasing of strain and tends to be constant after a peak value. The peak stress increases with increasing of strain rate and decreases with increasing of deformation temperature. Strain rate sensitivity value m of the UFG CP-Ti is found to increase with increasing of temperature and decreasing of strain rate. And the grains in the deformed samples are coarsened and the size of subgrains increases; the number of fine grains in grain boundaries also increases, indicating that the main softening mechanism of the alloy during hot compression deformation is attributed to the transformation from dynamic recovery to dynamic recrystallization