Fe-Mn-Al-C austenitic lightweight steels have been studied for potential applications in structural materials. They can reduce energy consumption while improving safety standards. The low density and high ductility of Fe-Mn-Al-C austenitic lightweight steels are well aligned with the requirement of future society. The excellent mechanical properties in the austenitic lightweight steel are obtained through the aging treatment in the temperature range of 450-550°C. Not only nano-sized (Fe, Mn)₃AlC (κ-carbide), which helps to improve strength in grain, but also the cellular structure is formed at the grain boundary during aging treatment. The fine precipitation of the intragranular κ-carbide dramatically increases the yield stress. However, the grain boundary cellular structure of κ-carbide+α-ferrite is significantly coarse. Therefore, it can cause significant ductility loss. The role of grain boundary precipitation behavior on the intergranular fracture was intensively studied. The chemical composition of the studied steel is Fe-22Mn-8Al-0.8C. The aging treatment was conducted at 550°C for different times up to 30000 minutes. An uniaxial tensile test was conducted at room temperature. The microstructure of each condition is analyzed using FE-SEM and FE-TEM. It was confirmed that the Mn content increased, and the Al content decreased at the grain boundary at the beginning of aging. As aging further progressed, a cellular structure of α-ferrite + κ-carbide was formed at the grain boundary. The fracture behavior changed from the ductile to the intergranular brittle fracture as the grain boundary was decorated with the cellular structure. The role of grain boundary precipitation behavior on the ductile to brittle transition was further discussed in austenitic lightweight steel.