Abstract: Optimizing the solution treatment process is crucial for enhancing the comprehensive properties of Fe-Mn-Al-C lightweight steels. In this work, the effect of solution treatment temperature on the microstructure and mechanical properties of an Fe-Mn-Al-C steel was systematically investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and tensile testing. The results reveal that the microstructure of the Fe-15Mn-8Al-0.25C lightweight steel consists of a dual-phase austenite-ferrite structure at all solution treatment temperatures. As the solution temperature increases, the grain size first refines and then coarsens, decreasing from 16.1 μm to 13.1 μm and subsequently increasing to 21.3 μm, while the austenite volume fraction and the proportion of high-angle grain boundaries decrease to 31.7% and 34%, respectively. After solution treatment at 1 000 ℃, the experimental steel exhibits excellent mechanical properties with a low yield-to-tensile ratio and a high product of strength and elongation. The fine grain size, appropriate austenite-to-ferrite phase ratio, and moderate proportion of high-angle grain boundaries are the primary reasons for its superior performance. These findings provide a theoretical basis for the chemical composition design and process optimization of dual-phase lightweight steels.