Abstract: Research activities of supercritical water reactor (SCWR) have been carried out worldwide, aiming at cost reduction by system simplification and higher thermal efficiency. One of the most important tasks for the design and assessment of SCWR performance is to develop system analysis codes which are applicable under supercritical conditions. The paper presents the development of new system analysis code ATHLET-SC based on ATHLET 2.1A. Thermo-physical properties package valid for supercritical conditions are implemented into the existing ATHLET code to extend its application to safety analysis of SCWR. In to evaluate the applicability of the modified code, a core calculation model of mixed SCWR (SCWR-M) was proposed and analyzed, and transients of core power were simulated. Moreover, a simplified supercritical water cooled loop was proposed and its stability behaviors were analyzed. The results achieved up to now indicate a good applicability of the modified ATHLET code (ATHLET-SC) in simulation of SCWR.