In the present paper, the SiC nanoparticles were added in interface joints of thick AA5083 aluminium plates. Double-sided friction stir welded (DS-FSWed) was used to study the influences of added nanoparticles on joint characteristics. Also, the effects of different parameters such as rotational and traverse speeds, pin lengths, and the number of passes were studied. The macro and microstructural features as well as fracture surfaces of tensile specimens were studied using optical and scanning electron microscopies (SEM), respectively. The mechanical properties (tensile and hardness tests) of the nanocomposite weldments and base metal were investigated. Furthermore, fracture toughness test was employed to study the resistance of the base metal and the optimum weldments to crack growth. The results show that by changing the rotational and traverse speeds as well as pin length, the defect-free nanocomposite joint can be achieved. The SiC nanoparticles were added to joint area caused to considerable decrease in the grain size as well as an increase in the hardness of stir zone (SZ) which were compared to the base metal and without powder sample. According to the results, under the optimum conditions for the nanocomposite welds (i.e., rotational speed of 80 rpm, traverse speed of 1000 mm/min, and the pin length of 6 mm), the distribution of the powders was uniform and a defect-free welded joint was obtained. In this case, the tensile strength and fracture toughness reached 84 % and 89 % relative to those of the base metal, respectively.