We report on a microfluidic trap array that separates and captures circulating tumor cells (CTCs) from whole blood. The device is a series array of microfluidic branches that utilizes the difference in flow rates between the bypass channel and the trap channel to allow CTCs in whole blood to be separated and trapped. Once a trap has captured a cell with diameter larger than the narrow trap outlet, additional cells arriving at the branch would flow towards the bypass channel due to its lower flow resistance. Results demonstrated that it was possible to capture CTCs from the whole blood of a mouse with full-blown metastasis. With further developments, the bypass integrated microfluidic trap array could become a useful tool for the early prognosis of cancer metastasis.