Display omitted •Two different ultrafine grained (UFG) microstructure were obtained using friction stir welding.•Effect of grain size reduction within the UFG regime on fatigue properties was studied.•Comparison of fatigue properties among UFG microstructure and fine grain microstructure of same composition was done.•Fractographic study to understand the crack initiation and propagation of all the three microstructure was done. The main objective of the present study was to examine the effect of grain size on the fatigue properties of 5024 Al alloy. Two ultrafine-grained (UFG) and one fine-grain microstructures were produced via friction stir processing (FSP). Fatigue properties were investigated using a fully reversible bending fatigue testing machine. Microstructural evolution during fatigue was analyzed by carrying out interrupted fatigue testing with subsequent orientation imaging microscopy. Emphasis was given to grain size on the evolution of deformed structure, intergranular dislocation density, and overall S-N curve. A comparison with literature demonstrates the importance of the processing route to obtain the UFG microstructure. This difference is highlighted through an analysis of Basquin equation parameters. The results indicate that FSP can be utilized as an effective route to obtain stable UFG microstructure. A fractographic study was done to understand the effect of microstructure on fatigue crack initiation and propagation rates. Crack growth rates in stage-II has been compared for the three different grain sizes.