The thermal stability of Mn12-AcMn12(CH3COO)16(H2O)4O1252CH3COOH54H2O magnetic molecular crystals was investigated by TG-DTA from room temperature to 270℃. According to the results of XRD and TG,we found that Mn12-Ac lost the crystal water and the acetic acid molecules of solvation which were located between the adjacent molecules in the temperature range from 25℃ to 1 10℃. At the same time, 4H2O which were linked with 4 Mn3+ were lost. The crystalline structure of Mn12-Ac single crystal was destroyed during this course. However, the core structure in the molecule was well preserved. From 180℃ to 230℃, a further transformatio n took place, Mn12-Ac lost all the CH3COO- ions and turned intoγ-Mn2O3, with a little amount of Mn3O4.