Circulating tumor cells (CTCs) in peripheral blood has clinical significance in highlighting disease status for cancer patients. Thus analyzing blood samples, a commonly extracted body fluids in any health test, can be very helpful in prognosis. Here, we present a microdevice that permits separation of CTCs and blood constituents via their distinctively different biorheology. No functional modifications of the device were required and viable CTCs were obtained. Further studies on viable CTCs might give insights into the nature of the metastatic process. In our microfluidic device, the efficiency for isolating MCF-7 cells (breast adenocarcinoma) was 83%. Comparing with normal cultures, reseeded MCF-7 cells from isolation show no difference in proliferation rate highlighting these cells remain viable. Preliminary results show great promise to efficiently isolate and enumerate viable CTCs from peripheral blood with unique biomechanical characteristics. The microfluidic device presents an effective means for CTC studies and monitoring of cancer.