Particles dispersion plays an important role in many in-dustrial applications such as combustion, pollution control and also in experimental measurements like Laser Doppler Velocimetry. In this last case, particles are supposed to have the same behaviour as fluid particles in order to give relevance to the experimental measure. However it has be shown Jacquin et al. (1991) that noticeable errors can appear in the rms velocity measurement of supersonic jet or shear layer, even if care has been taken concerning particle seeding of the flow. The aim of this paper is to use direct numerical simulation of particle-gaz flow to investigate this phenomenon.