In order to produce granules granulometrically polidispersed in a very little range, a fluid bed granulation process of the type comprising the steps of preparing a fluid bed of seeds (S1) of the substance to be granulated, having a free surface (P) substantially horizontal; and feeding a continuous flow (L) of a fluid comprising a growth liquid, provides in the fluid bed for a continuous vortex (V) with a substantially horizontal axis, in which an upper zone (Z1) of seeds wetting and evaporation of possible solvent contained in the flow (L) and a lower zone (Z2) of solidification/consolidation of the growth liquid are identified.