Abstract: System analysis code is important for the global simulation of the sodium-cooled fast reactor (SFR) system as well as transient and accident safety analysis. In this paper, the best estimate system code ATHLET for light water reactors, developed by Gesellschaft für Anlagen-und Reaktorsicherheit (GRS) in Germany, was modified for SFR application. Thermal-dynamic and transport properties as well as heat transfer correlations for sodium were implemented into the ATHLET code. The modified code was then applied to simulate the Phenix reactor in France, and validation of the code was conducted with the Phenix reactor natural convection test. The calculation results were compared with the test data. The results show that the modified ATHLET code has good applicability in simulating SFR systems.