To investigate the effects of grain size and processing temperature on the deformation behaviors of commercially pure titanium (CP-Ti), the tensile tests were conducted at room temperature (RT) and liquid nitrogen temperature (LNT) on the samples with the average grain size of 2, 9, 23, and 51 μm, respectively. The experimental results based on EBSD and TEM characterizations showed that dislocation slip was the main deformation mechanism at RT, while abundant deformation twins including {10 2} extension twins, {11 } compression twins and some {11 }-{10 2} secondary twins were observed under LNT condition. The transition of deformation modes from dislocation slip to dynamic twinning contributes to the outstanding mechanical properties of the samples with a larger grain size at LNT. Additionally, a modified Hall-Petch relationship was proposed to study the quantitative contribution of average grain size and deformation twins on yield stresses of CP-Ti under LNT condition.