High temperature design and evaluation of creep-fatigue damage for sodium-to-sodium heat exchanger, decay heat exchanger (DHX), in a sodium test loop have been conducted. The DHX is a shell- and tube-type heat exchanger with an outer diameter of 21.7 mm and effective length of 1.73 m. The DHX shell and tube materials were Mod.9Cr-1Mo steel. The temperatures of shell inlet and shell outlet in the DHX are 510 °C and 308 °C, respectively, while the temperatures of tube inlet and outlet are 254 °C and 475 °C, respectively. Three-dimensional finite element analysis was conducted for the DHX, and evaluation of creep-fatigue damage at several critical locations of the heat exchanger was carried out according to the elevated temperature design codes of the ASME Section III Subsection NH and RCC-MR. Evaluations on the integrity of the DHX and code comparisons were carried out for the critical locations of the DHX. The evaluation results showed that damages at the tubesheet joints of tube-to-tubesheet and tubesheet-to-shell were not so critical in the present DHX model.