Abstract: The preheated and non-preheated 4 mm China low activation martensitic steel(CLAM) plate were welded by TIG welding and post-weld heat treating, and the hardness and microstructure in welded joint were tested and observed.The results show that the hardness in the weld metal is higher and the softening band appears in the heat-affected zone(HAZ) closing to the base metal.The tempered lath martensite is observed in welded joints.There is no significant difference in martensite content between the preheated and non preheated weldments.A large amount of carbide particles are observed in the grains and at the grain boundary.The carbides are small rods in weld metal and large granular in the HAZ and base metal respectively.The microstructure and carbides in welded joints have great effect on the hardness.