The separation equations for the enrichment of heavy water in the continuous-type thermal-diffusion columns have been derived by treating the enriching and stripping sections separately. The experimental works for the separation of H2O-HDO-D2O system have been also conducted. The present theoretical predictions are in better agreement with the experimental results than those predicted in previous works, in which the average value of concentration-product term was taken as one at the feed position, while in the present study it was made separately in enriching and stripping sections.