In this paper, we report the development of a simple inertial microfluidic device with a serpentine channel for efficiently separating blood cells from plasma. The working mechanism of this device relies on the two-sided secondary flow aided inertial focusing of particles in a serpentine channel. Specifically, blood cells were focused along two sides of the channel, while the blood plasma was collected at the cell-free region within the channel centre. The device was tested with diluted (1/20) whole blood. A relatively high flow rate of 350 μl min⁻¹ with a purity of ∼99.75% was achieved in a single process. A further improvement to 99.95% purity was obtained after a second process. Parallelization with eight parallel serpentine channels achieved a high flow rate of 2.8 ml min⁻¹ and a massive throughput of 7 × 10⁸ cells per min. Our device could be easily integrated with other sample preparation processes or detection units to form a sample-to-answer lab-on-a-chip system.