The separation equation for heavy water enrichment by thermal diffusion in countercurrent-flow Frazier schemes has been derived and the average value of concentration-product term was treated separately in the enriching and stripping sections. The effect of the flow-rate fraction variation of a countercurrent-flow device on separation efficiencies for a H2O-HDO-D2O system, has also been investigated theoretically and experimentally. A qualitative agreement is achieved between the analytical predictions and experimental results. The analytical predictions are represented graphically and are compared with that in a classical Frazier scheme of the concurrent-flow device. Considerable improvement in the device performance is obtained by using a countercurrent-flow Frazier scheme with flow-rate fraction variations, instead of using a concurrent-flow device.