Type 316 stainless steel with low-carbon and medium-nitrogen contents called 316FR stainless steel is a candidate structural material for reactor vessels and internals of future-generation fast breeder reactors (FBRs). The reactor vessel cannot be manufactured from rolled or forged steel, but can be built at reasonable cost by welding rolled steel plates. In this manufacture approach, the reliability of the welded joint must be indicated. Two types of filler metals are candidates for 316FR welded joints: types 316FR and 16-8-2 filler metals. The chemical composition of type 316FR filler metal is close to that of the stainless steel; type 16-8-2 filler metal contains lower amounts of Ni, Cr, and Mo than that of the stainless steel. This study evaluated the need to consider the welded joint strength reduction factors in 316FR welded joints under design of future-generation FBRs. To this end, the tensile and creep strengths of types 316FR and 16-8-2 weld metals were measured, and the effect of δ-ferrite in weld metals was evaluated in creep strength tests of 316FR welded joints. In tensile and creep strengths of 316FR welded joints welded by both metal types, the welded joint strength reduction factors were immaterial. The creep strength of 316FR welded joints was negligibly affected by δ-ferrite levels from 4.1 to 7.0 ferrite number (FN) in the Welding Research Council-1992 diagram. Furthermore, the tensile and creep strengths of 316FR welded joints by two methods (gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW)) were the same. Therefore, the tensile and creep strengths of 316FR welded joints in above condition are ensured the reliability of similar to 316FR stainless steels.