The effect of heat input on microstructure and mechanical properties of friction stir welded pure copper was investigated. For this purpose, 5 mm thick copper plates were friction stir welded at different traverse and rotational speeds. Also, the peak temperatures of the joints during friction stir welding were recorded using accurate thermocouples. Moreover, the microstructure, hardness and tensile properties of the joints were examined. The results showed that the outline of the stirred zone were observed at lower heat input conditions but disappeared when heat input increased. The thermomechanically affected zone was clearly characterized by elongated and recrystallized grains, respectively at lower and higher heat input conditions. Moreover, the lower heat input conditions caused to finer grain sizes of the nugget zone, and hence higher hardness and tensile strength. The joints welded at higher heat input conditions had more elongation and showed more ductile fracture mode.