To study the hygroscopicity of cephalosporins, and get their critical relative humidity (CRH).The reagents were put in desiccators with different relative humidity.The moisture absorption was determined in different time.Then the curve chart of moisture absorption equilibrium was draw.The moisture absorption equilibrium time of cefathiamidine, cefazolin sodium, cefoperazone sodium, and cefuroxime sodium was 72 h.The CRH of them was 31%, 32%, 75%, and 73%, resp.The hygroscopicity study of cephalosporins can provide evidence of reasonable relative humidity for its production