To clarify the influence of particle size on dissolution behavior, relationship between the particle size and dissolution rate was investigated using three powdered sulfonamides, and following results were obtained. 1) At particle size below 300μ, the lines obtained by plotting dissolution rate versus time have positive intercepts and the values of intercepts increase qualitatively with decrease in particle size. At above 300μ, the lines pass closely through the origin. The fact indicates that the critical particle size is at about 300μ in the initial dissolution process. 2) Plot of the dissolution rate constants against rotating speed showed a straight line having an inflexion at 500 rpm (Nf) in spite of various particle sizes. Above Nf, the slopes of these lines do not vary with change in particle size but below Nf, these increase with decrease in particle size.