Abstract: It is well known that some pharmaceutical properties of oleagenous suppositories change during storage. To clarify these phenomena, accelerated storage experiments were performed by use of five local semisynthetic fatty bases and one foreign base. The melting points increased gradually to an extent of 2～3℃ when the bases were stored at 30±1℃ after 4～6 days. The softening time and the disintegration time were also prolonged. X-ray diffraction was measured, and it was confirmed that there were two different crystal forms in the semisynthetic bases. One of them was an unstable form (A-form) found in freshly prepared samples. The other was a stable form (B-form) found in stored samples. The change of pharmaceutical properties of suppositories well corresponded to the A to B transition. It was also recognized that the transition rate intensely depended on storage temperature. In order to ensure the quality of suppositories, it is suggested that the melting point of freshly prepared bases are controlled within 34～35℃.