In this study, significant microstructure and mechanical property gradients were observed along the transverse and thickness directions of as-welded friction stir welded joints of Fe–18Cr–16Mn–2Mo–0.85N austenitic stainless steel. These gradients resulted in the combined overmatching feature with higher strength but lower elongation than base metal. Further, post-weld heat treatment was shown to effectively weaken these gradients in the as-welded joints. Therefore, the elongation of the joint was increased companied with enhanced strength. The tensile performances of the overmatched joints were determined by the match factors and the proportions of nugget zone (NZ) in gauge length. Further, a reduction in the match factor only resulted in obvious increment in elongation of the joint. However, the best tensile properties of the joints can be achieved by simultaneously decreasing both the match factor and the proportion of the NZ in the gauge range.