Particle velocity is a very important parameter in kinetic spraying (or cold gas dynamic spraying). It is difficult to measure the velocity of a particle with supersonic speed at low temperature (lower than 500 degrees C). Thus, in many investigations only estimated values are used for evaluating coating processes. In this paper, the modeling of particle acceleration was reviewed, and the measurement of in-flight particle velocity in a kinetic spraying process was performed. Particle velocity and flux distributions from different process gas temperatures and pressures were investigated. The influences of process gas temperature and pressure on particle velocity were discussed. Characteristic of Al-Si feedstock deposition onto a mild steel substrate was described by comparing coatings structures with the in-flight particle conditions. The deposition behavior showed two critical particle velocities for Al-Si powder deposition onto a substrate and for particle-particle bonding. (c) 2005 Elsevier B.V. All rights reserved.