The combined effect of grain size (recrystallized grains of 34, 86, 105 and 128 mu m) and strain rate (0.01, 0.05, 0.25, 2.5 and 5 min(-1)) on the evolution of dislocation substructures and mechanical properties in oxygen-free high conductivity (OFHC) copper during room-temperature tensile testing has been studied. Under identical conditions of deformation, the flow stress values for smaller grain size were higher than those for larger grain sizes with the exception in the case of 86 mu m which has been attri buted to the in homogeneous substructural developments in the microstructures. The cell size decreases monotonically with increase in per cent strain indicating no signs of cell size saturation. The effect of strain rate on the development of dislocation substructures at constant strain is such that the cell size decreases initially but increases with further increase in strain rate for smaller grain sizes of 34 and 86 mu m while a reverse trend has been observed for larger grain sizes of 105 and 128 mu m. A graph of the cell size strengthening coefficient, k, and the strain rate shows three distinct stages in the curves for different grain sizes.