By using the entropy of information theory, a more reasonable separation efficiency can be defined from the point of view of statistics. This newly defined separation efficiency varies from zero for the case of non-separation, in which the mass fractions of valuable component in both product and residuum take the same values as that in feed, to unity for the case of ideal separation, in which the valuable material and useless material in the feed leave the separator as the product and the residuum, respectively, and has sufficient detection sensitivity for even a small change of mass fraction of components in the product or in the residuum. The newly defined separation efficiency, which satisfies the fundamental requirements described above and has clear physical meaning on the basis of its definition, may be useful to evaluate the performance of separation processes.