Cell separation based on size by microfluidic devices has become a widely studied research area to facilitate the diagnosis of malaria and cancer, in particular. Conventional diagnostic systems are sophisticated but expensive; however, with microfluidic devices a broad range of laboratory applications can be accomplished in a hand-held device. In this work, we fabricate a series of devices as Dean force coupled curved mirochannels for separation of human breast cancer cell lines which are MCF-7 (similar to 20 mu m) and MD-MBA-231 (similar to 15 mu m). These curved channels were fabricated in four different dimensions: 400 mu m wide x 81 mu m high; 500 mu m wide x 84 mu m; 600 mu m wide x 91 mu m high; 700 mu m wide x 86 mu m high. These channels have one inlet and three outlets. Each channel experienced different flow rates to observe cell focusing and separation. The MCF-7 cell line was labeled with carboxyfluorescein succinimidyl ester, which has a fluorescence characteristic, whereas the MDA-MB-231 cell line was unlabeled. Fluorescence microscopy experiments were performed to determine the appropriate flow rates for focusing. The investigation of cell separation yield was performed by flow cytometry. Adjusting flow rates revealed that enrichment of MCF-7 cells requires very high flow rates. Flow cytometry analysis confirmed enrichment to occur at the second outlet of each channel. The efficiency of enrichment was observed in the microchannel with 500 mu m width as high as 93 %. Our results suggest that these curved channels can be regarded as a prototype of a microfluidic diagnostic device due to their fast reaction time, relatively accurate results, low cost and miniaturized features.