A two-dimensional mathematical model was developed theoretically to predict the degree of separation of heavy water in an inclined flat-plate thermal-diffusion column. An orthogonal expansion technique for solving separation problems of heavy water enrichment under plate aspect ratio and inclination angle variations was carried out analytically. The effects of plate aspect ratio and inclination angle on the degree of separation have been investigated with a consideration of a fixed operating expense. Considerable improvements in device performance are obtained if the thermal-diffusion columns are inclined at the optimal angles as compared to the vertical orientation. Further improvement can be achieved if the flow-rate fractions of top and bottom products are suitably adjusted.