The effect of zone length on the separation efficiency in multipass zone-refining processes has been investigated theoretically. The optimal zone length for maximum separation efficiency in each zone pass has been determined. As a well known fact (Davies, 1959), the best performance in the first pass is achieved by normal-freezing operation. In multipass operations, the optimal zone lengths in each pass and their corresponding maximum fraction of solute removal were obtained with distribution coefficients k and number of passes n as parameters. The results showed the optimal zone lengths for all passes increase with distribution coefficient, but decrease with pass number.