Effects of inclined angle and plate spacing on the degree of separation in batch-type flat-plate thermal diffusion columns, with operating expense and fixed charge kept unchanged, are discussed. It was found that the degree ofseparation increases when the inclined angle increases or when the platespacing decreases. However, decreasing the plate spacing will lead to decreasingthe amount of solution to be treated in the column, while increasing the inclined angle will enlarge the operating time for reaching the steady state.