In this research dissimilar friction stir lap welding of aluminum alloy AA2024 and AA5052 was carried out and the rotation and travel speed of the tool was employed as process variants, whereas, the others (such as tool geometry, tilt angle, and etc.) were constant. Dissimilar FS welded joints were fabricated using rotation and travel speed of 800, 1000, and 1250 rpm and 50, 100, and 160 mm/min, respectively. The effects of these process parameters on the mechanical properties (tensile strength, tensile elongation, and hardness profile) and thermal cycle of dissimilar friction stir welded joints were investigated to find the optimum condition so that both tensile strength and tensile elongation have appropriate value. The microstructures of the lap joints were intensively examined by means of optical and scanning electron microscopy equipped with energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) pattern of base metal (BM) and stir zone (SZ) was used to investigate the texture changes after friction stir welding process. The result shown the optimum parameter will yield a defect free joint arisen from a suitable material ?ow and fine grain microstructure of stir zone; in the range of experiments, joint fabricated using rotation and travel speed of 1250 rpm and 160 mm/min, respectively, have the highest tensile strength and tensile elongation; whereas the rotation and travel speed of 800 rpm and 160 mm/min, respectively, have the lowest tensile strength and tensile elongation due to improper material flow and resultant void formation at the retreating side of the weld. The microhardness pro?les across the sections of the welds at the AA2024 show a general softening in the heat affected zone (HAZ) in contrast to the base material and nugget zone, while the hardness of the stir zone is slightly lower than the base metal, in contrast the hardness of the stir zone have higher value than the base metal at the AA5052. In all cases the microstructure of stir zones shown fully recrystallized, equiaxed, fine grain microstructure having about 4 ?m grain size due to dynamic recrystallization; whereas, the over aging and slightly grain growth have been observed in the heat affected zone. The EDS analyze results reveal the Cu-rich precipitates in the stir zone having very fine and uniform distribution. X-ray diffraction pattern show the texture change in the stir zone after friction stir welding process compared with base metal. Keywords Friction stir welding, Dissimilar lap joint, AA2024, AA5052, Microstructure, Mechanical properties.