Abstract: Application of the laser-based velocimetry to experimental researches of flows is closely involved in tracking ability of particles whose density or initial velocity is different from that of fluid.Based on the non-dimensional particle transient motion equation for high particle Reynolds number, a concept of velocity of small particles in response to arbitrary flows are pre sented and described mathematically.In various typical flows, the particle's transient motions are investigated numerically, and the effects of several parameters, such as particle diameter and density, on particle's velocity in responses to flows are analyzed quantitatively.