Theoretical conversion temperature was calculated between different crystal forms of nitrendipine from melting data. The different conversion conditions were investigated, such as high temperature, high temperature and humidity and high pressure. The theoretical conversion temperature between nitrendipine I and II is 158.88 ℃, that between nitrendipine I and III is 160.50 ℃ and that between nitrendipine II and III is 158.65 ℃, indicating nitrendipine polymorphs belong to a monotropic system. At high temperature, nitrendipine II and III both converted to I. Under high pressure, II converted to I. The results indicate that nitrendipine I was a stable form, while II and III were metastable form. The stability ranked as I＞II＞III.

Key words: nitrendipine, crystal polymorph, conversion, X-ray diffraction