The minimum air velocities required to keep particles from depositing on the bottom of a pipe are measured experimentally for 19 groups of spherical particles ranging from 20 to 1600 microns in three pipes of 20, 26 and 49 mm I.D. It is found that there is a substantial difference in the effect of particle size between fine- and coarse-particle suspensions. This difference is explained theoretically by means of numerical simulation on a digital computer based on the previously proposed ellipsoid bouncing model. A general correlation is presented to predict the minimum transport velocity over a wide range of particle size, in which an empirical equation for the distinction between the fine- and the coarse-particles system is included.