Glucocorticoids help animals respond to stressors but excessive glucocorticoids cause muscle atrophy, while insulin can promote anabolism and growth. In order to compare the glucocorticoids-induced ultrastructural changes between skeletal muscle and cardiac muscle, and investigate the preventive effects of insulin on the changes, eighteen male chicks with similar initial weight were randomly divided into three groups. The two test groups were respectively treated with high-dose dexamethasone alone or together with low-dose insulin by intraperitoneal injection, and the control group was treated with an equal volume of saline solution. The experiment lasted for ten days, and then the body weight, muscle size and ultrastructure in skeletal and cardiac muscles of twelve chicks were qualitatively or quantitatively analyzed. The results showed that high-dose dexamethasone induced obvious skeletal and cardiac muscle atrophy. The differences of ultrastructural changes between skeletal muscle and cardiac muscle (such as for the former or the latter, the intermyofibrillar-and-interfilamentary spaces reducing or enlarging, the mitochondria swelling seriously or enlarging lightly, the myofibril filaments compacting or loosing) suggested that dexamethasone induced skeletal and cardiac muscle atrophy by different mechanisms. Low-dose insulin did not affect the dexamethasone-induced decreases of body weight and skeletal muscle size, but alleviated lightly the dexamethasone-induced ultrastructural changes in skeletal muscle. Different from skeletal muscle, low-dose insulin almost resisted the dexamethasone-induced ultrastructural changes in cardiac muscle.