Friction stir welding (FSW) of electrolytic tough pitch copper plates was conducted using a conventional CNC milling machine. The microstructure evolution of the weld was correlated with the process parameters used in the study and in conjunction with increasing temperatures during processing. When the optimum process parameters were achieved, a sound weld joint was obtained. The weldments were evaluated by microstructural analysis, using optical and scanning electron microscopes, and in terms of mechanical properties. At early stages of FSW and/or when using less than optimum welding parameters low temperatures result, metal does not plasticize effectively producing defects, such as large cavities, porosity, and poor bonding, due to the lack of plasticized material. Cavities were found at the advancing region of the weld, and in this area the finest grains were observed from the entire weld. The cavities were reduced, and the grain size increased further along in the weld as the temperature increased also. The typical weld nugget found in the friction stir welding of other metals was not observed in this case. Dynamic recrystallization was observed in the “stirred zone” of the weld; considering that the strain rate in this region was the same in all three cases, the difference in grain size was attributed to the differences in process temperature.