According to classical electrodialysis (ED) theory, measuring limiting current (I-lim) with an assembled stack is a commonly used technique to decide operating current. The current/voltage method of recognizing limiting current does not always yield unambiguous estimates of the limiting condition when applied to a practical electrodialysis apparatus. Some researchers have suggested an arbitrary method of using current/voltage data. Considering that the aim of ED was to obtain high desalting efficiency, a new parameter regarding the optimal operating current has been defined as I-lim', and a new method to determine I-lim' was proposed to guide the operation of the ED process in this paper. This method consisted of measuring current as a function of desalting efficiency and determining the inflexion in a eta-I curve. To prove the feasibility of I-lim', the V-I, V/I-I and eta-I curves were plotted and compared by using NaCl aqueous solutions at different conductivities. The experimental results showed that I-lim' determined by the eta-I curve was exactly located between the two inflexions in the eta-I and V/I-I curves. A method of combining the V-I curve with the V/I-I curve was proposed to decide the range of operating current. Compared to the current/voltage method, this method is more readable and practical.