A Na-CO2 heat exchanger is one of the key equipments in the concept of coupling the supercritical CO2 Brayton cycle to the KALIMER-150 design. The exchanger is a shell and tube counter flow type with a hot sodium on the shell side and a supercritical carbon dioxide on tube side. In this study, a computational desigh methodology was developed and the sensitivity study was done on the major design parameters for the development of preliminary design. In the designing process, the design requirements are pressure drops and sodium maximum velocity in the shell side and aspect ratio of the heat exchanger. The evaluation revealed that the gap design of doubel well tube has noticeable effects on the thermal-hydraulic characteristics of the heat changer. Maintaining the operating condition to be identical to those of KALIMER-150, the heat transfer area of He packed case was 4 times moer than that of LBE packed case.