Abstract Various passive safety systems have been developed for advanced nuclear reactors. The use of natural driving force for passive systems requires accurate assessment of reliability and performance. Sensitivity analysis is an efficient technique to test the reliability and performance of a safety system, and in order to improve the computational efficiency, the adjoint-based sensitivity method has been introduced. The present study aims to develop an adjoint-based sensitivity method to evaluate the sensitivity of natural circulation system using high-Pr fluid, and to assess its applicability to the experimental facility developed in UNIST. The conservation equations for a closed rectangular loop are written, and the corresponding adjoint system is developed based on the Lagrangian approach. Three different functionals are evaluated which represent thermal-hydraulics and heat transfer performance with respect to various input parameters. The developed adjoint method shows reasonable accuracy in the sensitivity analysis compared with other methods, while significantly reducing the computational time. Following sensitivity analyses using various design parameters assess the sensitivity of natural circulation mass flux, temperature distribution, and Nusselt number. Three different system conditions are imposed to investigate the effects of (i) the implementation of temperature-dependent fluid properties, (ii) the orientations of the heat exchanger and (iii) the operating temperature range, on the entire system reliability. It is found that the variation of fluid properties with respect to the temperature has an important effect on the stability of heat transfer performance in the system. Finally, the advantages in using high-Pr fluids for natural circulation heat transport system is verified by comparing the sensitivities of Nusselt number for different fluids with different Prandtl numbers. The outputs of this study are expected to provide the assessment of the reliability and performance of natural circulation system using molten salt, which can be used for further design studies on molten salt system with low computational burden. Furthermore, this paper demonstrate the applicability of the adjoint-based method for molten salt system applications.