Polycrystalline aggregates consist of grains and grain boundaries and Plastic flow in polycrystals differ substantially from that of single crystals as some grains are oriented more favourably than others for effecting dislocation glide because there occurs a change in the orientation between different grains at the grain boundaries. So, these grain boundaries impart restrictions to the dislocation motion that leads to the greater strengths of polycrystals relative to single crystals, and the strengthening caused by the presence of grain boundaries is expressed by well-known Hall-Petch (H-P) relationship. The Hall-Petch relation is not just applied to the yield strength with the grain size but can also be applied to the flow stress at a different level of strain. The possible way to relate the flow stress with the grain size at different strain level is through the change in Hall-Petch parameters. Several models have been proposed in support of Hall-Petch relation but the significance of Hall-Petch parameters that keep the Hall-Petch relationship valid under the varying substructure with the level of deformation has not been properly understood. As these Hall-Petch parameters can be related to the grain interior and grain boundary strengthening and their variation can be related to the microstructural changes taking place in the respective region. In the present work variation in Hall-Petch parameters with strain is investigated for some FCC materials. Also, the variation in the dislocation density with strain is seen in the grain interior and grain boundary regions and the effect of their variation on the microstructural changes is discussed. Through this work, it was observed that the frictional stress increases in all the cases whereas in case grain boundary resistance three different behaviours were observed.