Inertial microfluidics has been attracting considerable interest in recent years due to immensely promising applications in cell separations and sorting. Despite the intense attention, the moderate efficiencies and low purity of the reported devices have hindered their widespread acceptance. In this work, we report on a simple inertial microfluidic system with high efficiency (>99%) and purity (>90%). Our system builds on the concept of two-stage inertial migration which permits precise prediction of particle or cell position within the microchannel. Our design manipulates the inertial equilibrium positions by modulating channel aspect ratio to achieve a complete separation. Here, we successfully demonstrate a complete separation of particles and isolation of rare cells in blood spiked with human prostate epithelial tumor (HPET) cells. Based on the planar structure, large separation spacing and predictable focusing, we envision promising applications and easy integration of our system with existing lab-on-a-chip systems for cell separations.