Background: Cold blood cardioplegic solution has been used to protect myocardium during open heart surgery with the hypothesis stating that it provides more oxygen supply to myocardium compared to crystalloid caridoplegic solution. We repeatedly infused cold blood cardioplegic solution to achieve myocardial protection. We biopsied a small portion of papillary muscle of patients with mitral valve replacement or double valve replacement during aortic cross-clamp time and evaluated the method of myocardial protection through the observation of changes in ultrastructure. We then analysed the relationship between changes in ultrasructure and peak postoperative CK-MB value and SGOT value. Material and method: We report observation on changes of myocardial ultrastructure, postoperative CK-MB and SGOT, and electrocardiogram in 31 patients who underwent cardiac operation. There were 11 males and 20 females, and they ranging in age from 28 to 69 years(mean score was 2.08$pm$0.560, it was 2.37$pm$0.558 at 40 minutes, and it was 2.36$pm$0.523 at 70minutes. Mitochondrial score increased significant at 40 minutes. Mean value of postoperative peak CK-MB and SGOT were 37.3$pm$17.061IU, 144.5$pm$125.5IU respectively. We were not able to find any new Q were in EKG after the operation. There was no significant relationship between myocardium mitochondrial score and mean value of postoperative peak CK-MB and SGOT. Conclusion: In conclusion, with this study the cold blood cardioplegic solution was incomplete in preserving ultrastructure of myocardium even with satisfactory results in serum enzyme and EKG evaluation.