Abstract: AIM: To investigate the protective effect of chitooligosaccharide (COS) on the cardiac muscle of rats with heavy load training. METHODS: Thirty six male rats (SPF, 3-months old) were randomly divided into three groups: normal control group, endurance exercise group and COS treatment group. Rats belonging to the endurance exercise group and COS treatment group exercised on the treadmill for 6 weeks. All rats were sacrificed and their hearts were quickly picked up 24 h after the last training. The morphological structure of the cardiac muscle of the rats was observed by light microscopy and transmission electron microscopy. SOD activity and MDA content of the cardiac muscle and blood serum were tested. SOD activity was tested by the xanthine oxidase method and MDA content was tested by TBA colorimetric condensation. RESULTS: SOD activity of the cardiac muscle in the COS treatment group (246.38±0.79) was significantly higher than the endurance exercise group (138.61±1.17) (P<0.05), but the SOD activity of the blood serum in the COS treatment group (67.92±18.47) was significantly lower than the endurance exercise group (98.79±16.10) (P<0.05). MDA content of blood serum and cardiac muscle in the COS treatment group (3.64±0.19 and 4.50±0.74) was significantly lower than endurance exercise group (6.87±0.34 and 7.86±0.46) (P<0.05). There were no significant differences in SOD activity and MDA content between COS treatment group and normal control group. There was significant myocardial cell injury in the endurance exercise rat group. The myofibril was blurred, broken and even necrosed and mitochondrial swelling and other morphological changes were observed by light microscopy. CONCLUSION: Heavy load training induced cardiac muscle injury and anti-oxidation functional change of the cardiac muscle and blood serum. COS may improve the anti-oxidant function of the cardiac muscle of rats with heavy load training and may maintain the normal morphological structure of the cardiac muscle.