A high-temperature natural circulation loop (NCL) using supercritical carbon dioxide as loop fluid is modeled to study the effects of operating variables and relevant design parameters on loop performance. The steady-state system model duly considers the axial conduction through loop fluid as well as loop wall and heat transfer with surroundings. The heat source is considered to be a heater with controlled heat flux and the heat sink is modeled as an end heat exchanger with water as the external cold fluid. The governing conservation equations for mass, momentum, and energy are nondimensionalized and are solved numerically discretizing in finite volume method. The numerical results are validated against experimental results reported in the literature in terms of modified Grashof number (Grm) and Reynolds number (Re). Results show that heat loss to the ambient affects the loop performance significantly for the high-temperature loop. It is also observed that the heat input at which the circulation becomes maximum can be increased by increasing either the diameter and/or the loop height. However, better performance is obtained with larger diameter tubes instead of longer loop heights. Axial conduction is seen to have a negligible effect on the overall loop performance. Boussinesq approximation appears to be reasonable as the operating conditions of the supercritical loop are away from the critical point.