In this work, we propose a rapid and continuous rare tumor cell separation based on hydrodynamic effects in a label-free manner. The competition between the inertial lift force and Dean drag force inside a double spiral microchannel results in the size-based cell separation of large tumor cells and small blood cells. The mechanism of hydrodynamic separation in curved microchannel was investigated by a numerical model. Experiments with binary mixture of 5- and 15-μm-diameter polystyrene particles using the double spiral channel showed a separation purity of more than 95% at the flow rate above 30 ml/h. High throughput (2.5 × 10(8) cells/min) and efficient cell separation (more than 90%) of spiked HeLa cells and 20 × diluted blood cells was also achieved by the double spiral channel.