Five polymorphic modifications of Cephradin were prepared by recrystallization from organic solvents. The isolated crystal forms were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and X-ray crystallography powder diffractometry. Modificaition 1 was the most stable form and decomposed at 201.3℃. Modification 3 and 4 were metastable. The dissolution of modification 3 and 4 was faster than that of marketed form.