The plastic deformation modes of commercially pure titanium (CP-Ti) were studied using an in situ tensile test monitored by electron-backscatter-diffraction (EBSD) assisted slip trace analysis. The plastic strain was primarily accommodated by prismatic slip, followed by deformation twins and pyramidal slip. The slip transmission between two adjacent grains was predicted using the geometric compatibility factor $m^{\prime }$, which influenced not only the degree of stress concentration but also the activity of dislocation slip systems. Stress concentration mainly occurred at GBs with an $m^{\prime }$ less than 0.5 and could be released by the activities of pyramidal slip or deformation twins with high critical shear stress (CRSS).