<p>In this study, heat pipe-based isothermal furnace liners (HPLs) employing sodium (Na) as a working fluid were fabricated and tested. The Na HPLs were constructed using two concentric stainless-steel tubes to form external dimensions of 101.6 mm × 70 mm × 410 mm (outer diameter × inner diameter × length). In particular, 1 mm thick porous stainless-steel wicks were directly sintered onto the interior surfaces of the HPLs to enhance manufacturability. The temperature uniformity of the Na HPLs was characterized in terms of the immersion temperature profile and the temperature uniformity factor of the working space over a temperature range from 800 K (530 °C) to 1100 K (830 °C). Two HPLs, filled with approximately 135 g and 175 g of sodium (corresponding to approximately 130 % and 170 % of the void volume of the wick, respectively), were compared to examine the influence of excess working fluid mass on the temperature uniformity. At 1100 K, the maximum temperature variations over a 20 cm vertical span from the bottom of the working space were 0.81 K ± 0.05 K and 0.63 K ± 0.31 K, with temperature uniformity factors of 0.57 K ± 0.02 K and 0.45 K ± 0.13 K, respectively. These results demonstrated that temperature uniformity was significantly improved at elevated temperatures due to the high sodium vapor pressure, and the excess working fluid mass had a limited effect on the temperature uniformity of the HPLs.</p>