Ibuprofen particles (mean particle size, 27 mu m and melting point, 76 degrees C) as core materials were directly coated with a water-soluble polymer. The primary particles were preserved using a composite fluidized bed with a dispersing mechanism at the bottom of the fluidized bed apparatus. Coated primary particles were obtained under the following 3 conditions: (1) Setting the spray air flow rate at 10 L/min from the initial to 2% coating, (2) adding the low-viscosity water-soluble polymer macrogol 6000 to the hypromellose coating solution, and (3) changing the spray air flow rate to 15 L/min from 2% coating. The particles obtained were confirmed to be coated primary particles by scanning electron microscopy of their cross sections prepared by the cryo-focused ion beam method. The dissolution test showed a marked improvement in the solubility of ibuprofen from the coated primary particles compared with that of a physical mixture. In conclusion, the optimization of the direct coating process made it possible to undertake primary particle coating of a raw material that has a low melting point and a particle size of not more than 50 mu m. Primary particle coating contributes to improvements in the physicochemical properties of drugs. (C) 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.