The effect of plate spacing on the degree of separation and production rate for the enrichment of heavy water in a countercurrent-flow Frazier scheme with N flat-plate thermal diffusion columns of same size and with total expense fixed, has been investigated. The equations for estimating optimum plate spacing for maximum separation and maximum production rate have been developed. Considerable improvement in performance for the enrichment of heavy water is obtainable when the thermal diffusion columns with optimum plate spacing are employed for operation. The fact that countercurrent-flow operation is more effective than concurrent-flow operation for the enrichment of heavy water in a Frazier scheme, is also confirmed.