Microstructural and mechanical properties of friction stir welded copper plates under different welding parameters including various transitional and rotational speed of tool and number of passes under argon as protective gas have been investigated in the present study. This research cleared that owing to the occurrence of recrystallization phenomenon in stir zone, fracture toughness and Vickers hardness can increase by 39.1% and 45.1% relative to base metal, respectively. While, occurrence of annealing phenomenon in HAZ leads to a downfall in microstructural and mechanical properties. Namely, the Vickers hardness 15.8%, the tensile straight 36.1% and the fracture toughness 14.3% decrease relative to the base metal. Furthermore, this study showed that for obtaining a better weld, it is suggested to perform the welding process in a condition that leads to lower heat transformation to workpiece instead of performance of two or more welding passes. Finally, it was cleared that the different conditions including rotational and transitional speed of tool and also, the number of welding passes can affect the amount of transferred heat to the workpiece, therefore, they can affect the microstructural and mechanical properties of obtained welds.