The thermal stability and mechanical properties of China low activation martensitic steel with Zr and Y were investigated via thermal aging at 550 ℃ for 8000 h.The Laves phase content monotonically increased with thermal aging,and the volume fraction of the Laves phases stabilized in the alloy after 3000 h of thermal aging.The observed degradation in mechanical properties was because of the coarsening of M23C6 carbides and matrix grains during the earlier stages of thermal aging.The precipitation of Laves phases and V3Zr3C particles increased the strength and hardness of the alloy.Grain coarsening was the primary reason for the decrease in impact properties,and the ductile-to-brittle transition temperature increased from -71 to-48 ℃ after 8000 h of thermal aging.