Defect-free welded joints were obtained by placing 316 L stainless steel at the retreating side (case 1) and the advancing side (case 2) of friction stir welds when joining China low activation martensitic (CLAM) steel to 316 L steel. Quenched martensite formed in the weld on the CLAM side, and the precipitates coarsened and aggregated in the outer HAZ, however, uniform austenite microstructures formed on the 316 L side of the weld. The microhardness profiles were asymmetric, and the maximum hardness zones located at the nugget on the CLAM side ranged from 350 to 450 HV. Softening in the outer HAZ on the CLAM side was observed, which was more pronounced in case 2. The welded joints had excellent tensile properties and always reached the tensile strength of the 316 L base metal. Better tensile properties of the welded joint were obtained in case 1, and the bonding strength of interfaces between CLAM and 316 L steels were always excellent in both cases.