The objective of this research is to investigate the effect of welding parameters on the microstructure and mechanicalproperties of friction stir (FS) welded butt joints of dissimilar aluminum alloy sheets between Semi-Solid Metal (SSM) 356-T6and AA6061-T651 by a computerized numerical control (CNC) machine. The base materials of SSM356-T6 and AA6061-T651were located on the advancing side (AS) and on the retreating side (RS), respectively. For this experiment, the FS weldedmaterials were joined under two different tool rotation speeds (1,750 and 2,000 rpm) and six welding speeds (20, 50, 80, 120, 160,and 200 mm/min), which are the two prime joining parameters in FSW. From the investigation, the higher tool rotation speedaffected the weaker material’s (SSM) maximum tensile strength less than that under the lower rotation speed. As for weldingspeed associated with various tool rotation speeds, an increase in the welding speed affected lesser the base material’s tensilestrength up to an optimum value; after which its effect increased. Tensile elongation was generally greater at greater toolrotation speed. An averaged maximum tensile strength of 206.3 MPa was derived from a welded specimen produced at the toolrotation speed of 2,000 rpm associated with the welding speed of 80 mm/min. In the weld nugget, higher hardness was observedin the stir zone than that in the thermo-mechanically affected zone. Away from the weld nugget, hardness levels increased backto the levels of the base materials. The microstructures of the welding zone in the FS welded dissimilar joint can be characterizedboth by the recrystallization of SSM356-T6 grains and AA6061-T651 grain layers.