High thermal and electrical conductivities, corrosion resistance and relatively good strength lead to use of copper and its alloys for several engineering applications. Copper alloys also find application in the nuclear industry for manufacturing storage canisters for spent nuclear fuel. Conventional fusion welding of copper and its alloys is difficult due to high-energy input requirement and various weld defects such as porosity, solidification cracks and distortion. Friction stir welding (FSW) has been proposed as an alternative way of joining copper as it is a solid-state joining process avoiding most of the fusion welding defects. Numerical models are very useful in understanding a welding process, and various models have been developed for FSW of steels, aluminum and titanium alloys. However, such models are not available for copper and its alloys. Here we present a three-dimensional heat transfer numerical model for FSW of copper. The model uses finite element method to calculate the three-dimensional distribution of temperature due to frictional heat generation at the tool and workpiece interface. The computed results are validated with the measured temperatures using a thermocouple near the tool shoulder. The experimentally welded samples are found to be defect free and acceptable based on radiographic testing. The tensile strength of these samples is measured and compared with strength of the base material. The variation in the peak temperature and weld joint strength is studied as a function of the rotation speed. This validated numerical model can be developed further by including material flow calculation for joining of copper.