This paper describes the preparation of ultrafine particle of insoluble drugs in water and the improvement of phyisico-chemical properties by using three types of fine grinding mills such as a planetary ball mill, a vibration rod mill, and a jet mill. Ursodeoxycholic acid (UDCA) was used as the model drug of insoluble drug. The samples ground by three mills were characterized by particle size distribution measurement, apparent solubility measurement, powder X-ray diffraction (PXRD). and in vivo test of a hindrance effect of reactive oxygen species (ROS) generation. The ground sample of UDCA with the vibration rod mill among the three grinding mills showed a high amorphous state and a high apparent solubility than other two mills, and the formulation prepared with ultrafine particle of UDCA decreased the reactive oxygen species generation and the lipid peroxidation in CCl4-induced oxidative stress mice. (C) 2004 Elsevier B.V. All rights reserved.