This paper aims to clarify the relationship between a) mean particle velocity at a specific point in a flow field, in which spherical powder with specific diameter range is seeded, measured by cross-correlation particle image velocimetry (PIV) and b) corresponding mean particle diameter, using one-dimensional model. The target spray method is cold spray with copper as powder material. Four distributions of particle diameter are tested. The particle velocity at the nozzle exit is calculated using a principle of cross-correlation PIV along the following steps; 1) twenty particles are distributed within the interrogation and search regions using random numbers for the first frame, 2) velocity of all particle diameters are calculated by one-dimensional model, 3) new particle locations after a specific time period Δt is obtained for the second frame, 4) brightness distributions for the two frames are obtained, 5) mean distance of particles' movement in the downstream direction, Δx, is obtained by crosscorrelation of two brightness distributions, 6) particles' velocity in the interrogation region is calculated as Δx/Δt. Then, Δx/Δt was compared with the particle velocity calculated by one-dimensional analysis for four mean diameters; number-/length-/ area-/volume-averaged diameters. The calculated results show that the arithmetic mean value of length-averaged and area averaged diameters provides closest velocity corresponding to the particle velocity obtained by cross-correlation PIV method.