Abstract: The effect of silicon on microstructure and mechanical property of China low activation martensitic (CLAM) steel was investigated. The tensile strength increases and the DBTT is higher but the toughness decreases by adding 0.2％ Si. The microstructure of CLAM steel appears as tempered martensite by light optical microscopy (LOM). The prior austenite grain size is smaller in CLAM steel with 0.2％ Si, which leads to the tensile strength increases and the toughness decreases. The tempered lath martensite is the primary microstructure feature by TEM. Two types of carbide are observed in CLAM steels. One is needle type and the other is grain type. The grain type carbide is identified as M23C6 carbide using selected area diffraction. The mantensitic lath width is smaller in CLAM steel with 0.2％ Si.