Abstract: A new method of separation and determination of ampicillin by high performance capillary electrophoresis (HPCE) is reported. By this method, the collected samples of ampicillin from different factories were assayed. The result of test shows that there are two chief polymers in the collected samples, i.e., an open cycle dimer and a closed cycle dimer, but the amounts of other polymers are very small. This method can be used to assay simultaneously the open cycle polymers and the closed cycle polymers in ampicillin, so it is a new method for the quality control of ampicillin.