Low-carbon/medium nitrogen 316 stainless steel called 316FR is a principal candidate for the structural material of a demonstration fast reactor plant. It is thus important to understand various effects on high-temperature strength of this steel. Especially the effect of grain size is important because the material with coarse grains will be used for thick components as well as weldment heat-affected zones. Various kinds of strength tests, including tensile, creep, fatigue and creep-fatigue tests, were conducted and the effect of grain size was investigated for both 316FR and 316 stainless steel with conventional chemical composition. 316FR showed considerably better properties than the conventional 316 with regard to creep and creep-fatigue strength. It was also found that the material with coarser grains have inferior characteristics in tensile, creep, fatigue and creep-fatigue tests. The applicability of Petch's equation was comformed for tensile and creep strengths of 316FR. A creep-fatigue life estimation method incorporating the grain size effect was proposed for 316FR.