This paper discusses key issues and highlighted topics in thermal hydraulics in connection with Japan's current sodium-cooled fast reactor development efforts, with particular focus on design study and related research of the Japan Sodium-Cooled Fast Reactor (JSFR). Several innovative technologies, i.e., compact reactor vessel, two-loop system, full natural-circulation decay heat removal, and recriticality-free core, have been investigated to reduce construction cost and to achieve a high level of reactor safety. Preliminary evaluations of innovative technologies to be applied to JSFR are ongoing. In this paper, the progress of design study is reviewed, and key issues are discussed. Then, research and development activities on the thermal hydraulics are highlighted in connection with the phenomena taking place in natural-circulation decay heat removal and innovative design and enhanced safety features.