The velocity of cold spray particles was measured by a diagnostic system for thermal spray particles based on thermal radiation. A laser beam was employed to illuminate the cold sprayed particles in cold spraying for obtaining a sufficient radiant energy intensity for detection. The measurement was carried out for Cu particles of different mean particle sizes. The particle velocity was also estimated using the previously developed two-dimensional axisymmetric model. It was found that the measured results agreed well with the calculated ones. The proposed measurement method in this paper is reliable. On the other hand, it is confirmed that the particle acceleration behavior in cold spraying can be accurately predicted through the simulation method developed previously. The optimization of cold spray process can be conducted following the simulation method.