Deformation-based particle/droplet separation is important in many industrial and medical applications. The roles of different physical parameters of particles/droplets such as viscosity, velocity, and size in the sorting process, however, remain elusive. Here, we designed a microfluidic device with a cylindrical post that can separate droplets depending on droplet size, viscosity, and velocity. We showed that droplets with a large size or low deformability (i.e., high viscosity or low velocity) were separated to side outlets in the microfluidic device, whereas droplets with a small size or high deformability exited to the center outlet. With high-speed imaging, we further identified two sequential droplet deformations during the sorting process and showed that the characteristic distance (delta) and the impact angle (theta), which were determined by the physical parameters of droplets, played a regulatory role in deformation-based droplet sorting. Droplet sorting to the side outlets occurred only when delta >= 0.542 or theta >= 28 degrees.