AbstractSteady state isoelectric focusing was treated as zero velocity isotachophoresis. From this standpoint, the author's theory of isotachophoresis was modified to deal with isoelectric focusing of protein molecules using carrier ampholytes, and mathematic simulations according to the modified theory were performed to calculate pH, voltag and concentration profiles in steady state isoelectric focusing. The results were in agreement with experiments with regard to resolving power for protein separation.