: The mechanical properties of the welded joints of the 2024 aluminum alloy have been studied by many researchers, although, few researches investigated the welding of nano and bimodal (nano and ultrafine grained) structures. Recently, bimodal structures with excellent mechanical properties have been noticed significantly. In this study, 2024 aluminum alloy with bimodal structure was used for friction stir spot welding, and the mechanical properties and microstructure of the weld were investigated. In order to achieve the bimodal structure in the 2024 aluminum alloy, the solid solution treatment and cyro rolling process to reduce thickness up to 85% was utilized. Then, to improve strenght and ductility, aging heat treatment (6 hours at 160 ° C) was carried out. Afterward, a bimodal structure containing nanoscale and submicron grains with strengthening S phase precipitates (Al 2 CuMg) was obtained. The results show that, the ultimate tensile strength was 39% improved. For mentioned sample, the hardness and elongation were 170 Hv and 11% respectively. Using mentioned process for the 2024 alloy, reveals improving strength and ductility simultaneously. Mechanical tests shown that Welding on cryo rolled alloy, cryo rolled and aged and 2024-T3 (with normal grain size) with rotational speeds of 1360 rpm, plunge depth of 2.3 mm, plunge rate of 2 mm / min and dwell time of 10 seconds as optimal conditions. The Welding zone of samples reveal fine and equiaxed grains because of the occurrence of dynamic recrystallization. In this case, the maximum failure load was 5280 N for the cryo rolled sample. Maximum failure load for cryo rolled and aged and 2024-T3 samples were 4830 and 4690 N respectively. Increasing the strength of the cryo rolled alloy was attributed to the finer grain size of the welding zone and the formation of S-phase during the welding process. Keywords: Bimodal structure, Aluminum 2024, Cryorolling, Friction stir spot welding
