The effects of plate aspect ratio on the degree of separation and production rate in a countercurrent-flow Frazier scheme with N flat-plate thermal-diffusion columns of same size and with plate surface area fixed, have been investigated. The equations for estimating optimum plate aspect ratio for maximum separation and maximum production rate have been derived. Considerable improvement in performance is obtainable when the thermal diffusion columns with optimum plate aspect ratio are employed for operation. The fact that countercurrent-flow operation is more effective than cocurrent-flow operation in a Frazier scheme, is also cinfirmed.