The solid-solid transition of phenylbutazone polymorphs was studied by heating, differential scanning calorimetry (DSC) and X-ray diffractometry. The difference in dissolution rate due to the polymorphism was confirmed. Three polymorphs of phenylbutazone were observed. Form I melted at 103.degree. C, Form II twice at 93.degree. C and at 103.degree. C, and Form III at 93.degree. C. The mutual transition phenomenon by heating among these three polymorphs was observed by DSC. Analyzing the dissolution curves of Forms I and II obtained by the stationary disk method and plotting the saturated concentration of the respective forms, the transition temperature estimated was 98.degree. C and the heat of transition 210 cal/mol. Form III was transformed to Form II through compression above a certain compressional pressure and to the intermediate form of Form II and Form III (designated as pseudo-Form III) under the compressional pressure 1 ton/cm2. The intrinsic dissolution rate of Form III was not obtained. The dissolution rate of pseudo-Form III was faster than any of Form I and II. If the transition of Form III could be suppressed, more soluble preparations of phenylbutazone would be available.