Abstract: Based on the new Chinese low activation martensitic (CLAM) steel being developed as for fusion reactors, the effect of silicon adding on the mechanical properties of 9Cr-1.5WVTa steels was investigated. The results show that adding 0.2% silicon leads to higher tensile strength, and more excellent plasticity and impact toughness. The ductile-brittle transition temperature (DBTT) is decreased from -13 ℃ to -30 ℃. The 9Cr-1.5WVTaSi steel has a smaller prior-austenite grain size, but microstructures are similar, both fully martensitic steels free of δ-ferrite. It was summarized that the improved tensile and impact properties of the 9Cr-1.5WVTaSi steel were attributed to adding 0.2% silicon, refined the prior-austenite grain size.