Cryogenic distillation experiments have been performed with an H-D-T system (1.5 g of tritium) by using two columns with different inner diameters (1 and 2 cm) and different size packings (a 3-mm Dixon ring for the larger column and a 1.5-mm Dixon ring for the smaller column). The height equivalent to a theoretical plate (HETP) values measured during total reflux operation ranged from 4 to 6 cm for both columns and showed no apparent dependence on vapor velocities. The HETP values under total recycle operation, which varied from 3 to 6 cm, gradually decreased with increasing reflux ratios. The feed flow rates also had influenced the HETP. Consequently, the HETP values were correlated by the flow rates of the feed and vapor streams of the column and by the reflux ratio.