The effect of the variation of column heights on thermal diffusion performance along the multi-column device for recovery of deuterium from H–D gas mixture, has been investigated with the total sum of column heights fixed. The equations, which may be employed to predict the optimal column height ratio for the maximum recovery, have been derived. Considerable improvement is obtained if the column heights properly increase at a constant ratio for a certain flow rate operation, instead of using the uniform column height device, with the same total sum of column heights.