We investigate the key parameters that influence the efficiency of the previously presented dielectrophoresis(DEP)-activated cell sorter (DACS). While the conventional cell separators typically sort cells depending on the different sizes of cells, our system differentiates similarly-sized cells by responding to the variations of electrical properties among cells namely conductivity and permittivity. Thus, continuously varying DEP forces generated by the system enable us to separate MCF10A (normal cells derived from human breast epithelium) and MCF7 (human breast cancer cells) cells of similar size. Unfortunately, however, both cell types possess a wide range of cell size variation. In previous study, therefore, separation efficiency was limited to 77±5% for sorting MCF10A cells and MCF7 cells. To analyze separation efficiency according to cell size, we calculate cell size distributions before and after separation and then analyze the separation efficiency of cells according to cell size. Although cell separation is performed relying on the electrical properties of the cell, we conclude that cell size remains one of the key parameter that determines the efficiency of cell separation.