Inertial microfluidics is a promising approach for particle separation because of the superior advantages of high throughput, simplicity, precise manipulation, and low cost. However, the current obstacle of inertial microfluidics in biological applications is the broad size distribution of biological microparticles. Most devices only work well for a narrow range of particle sizes. For focusing and separating a new set of particles, troublesome and time-consuming design, fabrication, testing, and optimization procedures are needed. As such, it is of particular interest to design a microfluidic device that can be tuned and adjusted to separate particles of various sizes. This paper reports on the proof of concept for a stretchable microfluidic device that can control the length via a stretching platform. By changing the channel dimensions, the device can be adapted to different particle sizes and flow rate ratios. We successfully demonstrate this approach with the separation of a mixture of 10 and 15 μm particles. Stretching the device significantly improves the focusing and separation efficiency of the specific particle sizes. We also show that there is an optimum stretching length, which results in the best separation performance. The proof of concept reported here is the first step toward designing stretchable inertial microfluidic devices that can be implemented for a wide range of biological and medical applications.