In the present study, the effect of tool rotational speed on microstructure and mechanical properties of friction stir welded joints between commercially pure copper and 6351 Al alloy was carried out in the range of tool rotational speeds of 300-900 rpm in steps of 150 rpm at 30 mm/minutes travel speed. Up to 450 rpm, the interface of the joints is free from intermetallics and Al4Cu9 intermetallic has been observed at the stir zone. However, Al4Cu9 intermetallic was observed both at the interface and the stir zone at 600 rpm. At 750 and 900 rpm tool rotational speed, the layers of AlCu, Al2Cu3 and Al4Cu9 intermetallics were observed at the interface and only Al4Cu9 intermetallics has been observed in the stir zone. The maximum ultimate tensile strength of ~207 MPa and yield strength of ~168 MPa along with ~6.2% elongation at fracture of the joint have been obtained when processed at 450 rpm tool rotational speed.