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Movement of dislocations and their interaction with grain boundaries during yielding process of metal polycrystals are numerically simulated . Simulation results show that when the size of FR source is close to grain diameter, the resolved shear stress needed to emit a dislocation loop increases sharply with decreasing grain size. This effect is introduced into the critical resolved shear stress, which is used in the continuum mechanics-based crystal plasticity analysis. Results of the analyses of polycrystal models show distinct increases of yield stress for specimens with smaller mean grain diameter. Evolution of GN dislocations in fine- and coarse-grained polycrystals are discussed.