In order to investigate the microstructure and mechanical property evolution of low-carbon reduced activation ferritic/martensitic (RAFM) steel during isothermal aging, the normalized and tempered specimens were aged at 600����C for 500, 1000, and 3000��h, respectively. The microstructural evolution with aging time was analyzed, including the precipitation and growth of M23C6 and MX-type carbides as well as the formation of Laves phase. The results indicate that the coarsening of M23C6 is more obvious than that of MX with increase in aging time. During the long-term thermal exposure, the Fe2W Laves phase precipitates adjacent to M23C6 along the prior austenite grain boundaries and packet boundaries. Lower carbon content can delay the precipitation of Laves phase compared to the steel containing higher carbon. In addition, the Laves phase precipitated along boundaries can provide the precipitation strengthening, slightly increasing the tensile strength of low-carbon RAFM steel after aging for 3000��h.