Discrete dislocation dynamics simulations are carried out to systematically investigate the rate dependent deformation behaviour of polycrystalline bulk copper by varying the loading rate in the range of 100–25,000 s 1 under tension. The underlying material model not only incorporates the realistic definition of nucleation time but also put emphasis on the role of obstacle density and their strength on dislocation motion. In the simulations, plasticity aris- es from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. The numerical results show that the rate sensitivity of yield stress in bulk polycrystals is controlled by the density of Frank-Read sources, obstacles and their strength.