Inertial microfluidic systems have been widely used for particle or cell separation applications, especially for rare-cell enrichment and separation from blood due to the high throughput and simplicity of the systems. However, most of the separation techniques using inertial microfluidic systems require dilution of blood samples or RBC lysis to achieve a high separation efficiency, which can adversely affect the throughput and/or analysis of the collected sample. We developed a cell separation technique compatible with untreated whole blood by inflection point focusing, that is, an inertial focusing phenomenon toward inflection points of the velocity profile. We generated a strong shear-gradient lift force by varying the velocity profile in a channel cross-section with the combined effect of the channel geometry and the co-flows of two liquids with different viscosities. The natural viscosity of blood leads to a lagging flow region in the middle of the channel where larger particles and cells can be extracted to the side flow region, which enables a highly efficient separation scheme with an unprecedented high throughput.