In this research, in order to eliminate poor mechanical properties of austenitic 304L stainless steels and utilize useful capabilities of these alloys in industry, firstly, an ultrafine grained/ nanostructured AISI 304L steel was produced by martensite thermomechanical processing. The alloy was then welded by resistance spot welding method. The samples were prepared in dimensions of 80*40*10 mm and then cold rolled to 90% reduction in thickness. Subsequently, to get ultra-fine/fine grained austenitic structure, the samples were annealed at 700°C and 900°C for 5 to 360 minutes. The produced samples containing various grain sizes, were resistance spot welded with different parameters of current 6 to 35 KA, electrode force 4 KN and welding time 6 to 18 cycle (0.1 – 0.3 s). For phase analysis, X-ray diffraction (XRD) and feritscope techniques were applied during cold roll and annealing process. Investigation of microstructures was carried out using optical microscope as well as scanning electron microscope (SEM). Electron backscatter diffraction was used for studying weld microstructure. XRD pattern of rolled samples exhibited the transformation of austenite to martensite while for annealed samples, it was reverse. Microstructure of annealed samples showed that annealing at 700°C for 120 minutes leads to ultra-fined / nanostructured with grain size of 400 nm and also, annealing at 900°C for 5 and 180 minutes leads tow steel with grain size of 3 and 12 micron, respectively. Tensile strength of ultra-fine sample, its elongation and hardness were 1060 MPa, 48% and 330 VHN, respectively. Studies of weld nugget structure obtained by EBSD, proved the existence of casting structure around the weld spot via formation of austenite matrix in addition to 3% delta ferrite phase. During shear tensile test, the fracture occurred in two distinct modes including interfacial and/or pullout failure. The pullout failure is the most ideal type of failure in tensile shear test. The most tensile shear peak load of weld spot of ultra-fined samples was 12000 N while its hardness was 250 VHN, obtained through welding with 30 to 35 KA currents, 18 cycles (0.3 s) welding time and 4 KN (80MPa) electrode force. The Results for different welded samples which had various grain sizes, showed that by increasing of grain size, the fracture load decreases. Also, the hardness of ultra-fined sample in comparison with other samples which had grain sizes of 3, 12 micron and as-received steel was more. Studying the fracture surfaces of the weld nugget in both cases showed that the occurred failure was a ductile failure with the formation of dimples. Tensile direction of dimples in line of applied force showed that the failure of the weld nugget had a shear (tensile) nature. Keywords : 304L stainless Steel, Ultra-fine Grained/nano structured, Resistance Spot Welding, Advanced Thermomechanical Process, Electron Backscatter Diffraction (EBSD
