We report the development of a simple poly(dimethylsiloxane) microfluidic device for high-efficiency trapping and sorting of micron-size particles. In this device, hydrodynamic fluid flow through the sieve-like microfluidic channel sequentially fills the trap positions with particles of the trap size, and particles smaller than the trap size pass through the sieve and are trapped by smaller traps downstream. By incorporating side channels alongside the main channel, we were able to decouple the fluidic flow in one stage from the flows in the other stages. This decoupling allows us to modularize each stage of the device regardless of the size of the entire device. In our demonstration experiment with the prototype, we showed that more than 85% of the polystyrene microspheres (of sizes 15 μm, 6 μm and 4 μm) were sorted in the correct segment of the device that targets their respective sizes. Moreover, this high-efficiency device was able to trap all microspheres which were introduced into the device. Finally, we tested the device's ability to trap and sort three different species of waterborne parasites (Entamoeba, Giardia, and Cryptosporidium) and obtained excellent sorting performance.