In present study, the effect of heat treatment after friction stir welding of dissimilar welds T6-7075 and T4-2024 aluminum alloys were investigated. Friction stir welding was performed at a constant welding speed 32 mm/min and variable rotation speed in the range of 290 rpm to 1140 rpm. After welding samples are taken under various heat treatment processes at different aging temperature and time period. Microstructural observations and phase analysis characterization was performed using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy ( EDS ) on welded before and after heat treatment. The mechanical properties were obtained by tensile and micro-hardness test in cross section of welds joint. The results showed that post-weld heat treatment causes abnormal grain growth turns destructive effect on the mechanical properties, while formation of fine and uniform precipitation recovery strength and ductility of welds joints. It is found heat treatment based on T6-7075 and T6-2024 procedure has highest and lowest impact on the restore of weld strength. Tensile test indicate that fracture occurred on the interface between TMAZ and HAZ in retreating side (7075) at as-weld joint, if that failure happens in the stir zone by applying PWHT. Surface fracture suggested fractures in PWHT samples are predominantly inter-granular, while in as-weld joint the fractures of joints are mostly trans-granular.