Abstract: Quasi two dimensional large scale vortex structure has a dominant effect on development of plane mixing layer.Crowe used Stuart's Vortices to simulate particle dispersion in free shear flows.The results of his simulation show that the experimental observations indicating particle dispersion rates greater than the fluid dispersion rates in free shear layer could be attributed to the effect of large scale vortex structure on particle dispersion.It is the incapability of Stuart's vortices of simulating development and merging process of these large scale vortex structure,that compromises the generality of his results.In this paper,we studied the particle dispersion in the mixing layer by using the random vortex method to simulate the creation,development and merging processes and the same results as Crowe's are obtained but in a more general sense.