The deformation behavior of ultrafine-grained (UFG) commercially pure (CP) Ti was investigated. The strain rate sensitivity and activation volume were calculated from strain rate jump tests performed at a temperature range of 250-450 °C and strain rate of 1×10-4 - 1 s-1. The results show that flow softening effect in steady state of deformation occurs in UFG CP Ti which is controlled by high-angle grain boundaries and dislocation activity during the deformation process. Strain rate sensitivity of UFG CP Ti is relatively low in value and increases with the rise of temperature. The activation volume of UFG CP Ti is also low in value and invariant with temperature. The values of strain rate sensitivity and activation volume indicate that dislocation-dislocation interactions within the grain interiors take place scarcely and interactions between dislocation and grain boundary can significantly affect plastic deformation of UFG CP Ti.