The present paper describes safety goals and principles for Generation IV energy systems, with emphasis on prevention and mitigation against severe accidents in the safety design corresponding to Level 4 of the defense-in-depth architecture. Consistent with them, a deterministic safety design approach has been applied to the Japan sodium-cooled fast reactor (JSFR) with the complementary use of a probabilistic approach. The JSFR safety design principle has also been developed with safety design features corresponding to essential safety functions, such as reactor shutdown, decay heat removal and containment. This concept especially highlights passive safety features and mitigation measures against core disruptive accidents. Design principle against the chemical activity of sodium is also discussed both on isolation from the reactor core safety and the contribution to the plant reliability.