The aim of this work is to optimize the relative and the absolute Critical Resolved Shear Stress (CRSS) of slip mechanism in α-phase of Ti-6Al-4V titanium alloy. The influence of grain size is then modeled through a local Hall-Petch relationship. A slip trace analysis technique coupled with statistical reasoning were used to identify the CRSS ratios of basal a , prismatic a and pyramidal c + a slip systems. The multiscale transition rule of Berveiller-Zaoui was then used to determine the absolute CRSS in three different microstructures; Ti-6Al-4V with ultra fine grains (UFG), fine grains (FG) and standard grains (SD). Finally, the local Hall-Petch relationship was optimized. As expected, plastic deformation is mainly accommodated by prismatic and then basal slip systems. Due to their high CRSS, sliding in pyramidal systems is more difficult. Grain size shows a significant role on the activation of slip systems. By increasing the grain size, the CRSS of each slip system type decreases and thus sliding becomes easier in coarse grains.