Abstract: Thirty SD rats were randomly divided into three groups:（1） treatment group:cortisone acetate was given intramuscular injection ata dose of 10mg per kilogram of body weight daity for 14 days,（2） pair-eating group: the amount of daily food intake was equal to that of treatment group.（3） free-eating group, The diaphragm function and morphology of rats were free-eating group. The diaphragm function and morphology of rats were examined after treatment.The results showed that cortisone may cause decrease of food intake, and diaphragm weight and serum albumin of rats. In treatment group, diaphragm fatigue occured after a high frequency electric stimulation, and light microscopy examination revealed degeneration, necrosis and atrophy of diaphragm muscle fibers. Pair-eating and free-eating groups did not show such changes altho-ughpair-eating group rats had a decrease of food intake,too. The relationship between malnutrition and the pathogenesis of the diaphragmatic myopathy due to corticosteroids was discuused.