Abstract: This paper reports studies on the relationship between polymorphism and stability of ampicillin sodium(69-52-3) (Ⅰ) for improving its property in solid state, and providing an easy and precise method to predict the shelflife of drugs.The three polymorphie modifications, A, B and C, and an amorphism of (Ⅰ) were analyzed by X-ray diffraction, IR, DTA, polarizing microscopy, electronscanning microscopy and NMR. The modifications A, B and C are crystallized from iso-propanol, dichloromethane and acetonitrile systems, respectively, while the amorphism is produced either by freeze drying or by spray drying.By means of the moisture-absorption test in RH: 52% and the thermal stability test, it was dynamically demonstrated that the modification C is far more stable than the modifications A and B, and the amorphism is the least stable. Through determining the heat of solution, it was found that (Ⅰ) in any Crystal modifications possesses less internal energy than the amorphism. Due to this, it was thermodynamically proved why the crystal modifications are more stable than the amorphism. In the thermal stability test, the method known as the rule of initial equal speed was used, thus the shelflife of drugs can be easily predicted without predetermining the order of reaction.Thus, it may be stated that improving the stability of (Ⅰ) in solid state can be achieved by changing its crystal modification.