Deuterium is the nuclear fuel for fusion reactors which may be created to replace fission reactors in the near future. Thermal diffusion is a powerful process for separation of hydrogen isotopes because of the large ratio in molecular weight. The equations which may be employed to predict the optimum angle of inclination for recovery of deuterium from water-isotopes mixture in inclined flat-plate thermal diffusion columns with transverse sampling streams, have been derived. Substantial improvement in performance can be achieved if the column is tilted at the optimal angle, instead of being placed vertically, so that the convection strength can be properly reduced and controlled, resulting in suppressing the undesirable remixing effect while still preserving the desirable cascading effect. The regions within which inclination improves the performance have been also delineated.