A rotating fluidized bed granulator/coater is a promising fluidization process in which microgranulation and coating of fine particles classified into Geldart's group-C can be successfully achieved. However, overwetting of the particle bed due to poor particle mixing in the rotational-axis direction has been a problem. This paper presents a new approach to improve the particle mixing in the rotational-axis direction, in order to prevent the particle bed from overwetting in the rotating fluidized bed wet granulation and coating. Inclined injection of the fluidizing air through an air distributor was proposed to improve the particle mixing in the rotational-axis direction. A computer simulation using a coupling model of a discrete element method and computational fluid dynamics was also conducted to analyze how the particle motion and mixing property are improved by the inclined injection of the fluidizing air. The simulation results revealed that a unique vortex particle flow along the rotational axis direction was induced in a rotating fluidized bed by inclining the inflow direction of the fluidizing air. This vortex particle flow led to a significant improvement of the particle mixing in the rotational-axis direction. Based on the simulation results, a new air distributor with inclined holes was made and its performance to avoid overwetting was evaluated by actual experiments of the particle mixing and wet granulation of pharmaceutical powders. The experimental results confirmed that the particle mixing in the rotational-axis direction was improved when the new air distributor with inclined holes was used. The large agglomerates generated using the conventional air distributor were no longer formed using the new air distributor with inclined holes because of better dispersion of the sprayed binder liquid in the particle bed, which resulted from an improvement of the particle mixing in the rotational-axis direction by the inclined injection of the fluidizing air. (C) 2013 Elsevier Ltd. All rights reserved.