Retrograde myocardial protection plays an important role in cardiac surgery and is widely used. We herein report a rare cardiac surgical case complicated with small coronary sinus ostium in which the cannula of retrograde cardioplegia could not be inserted. A 58 years old man was referred for the treatment of regurgitation and aortic regurgitation. Preoperative ECG gated computed tomography (CT) showed that the orifice of the largest coronary sinus was located in the right atrium with a diameter of only 4 mm with an other 3 smaller orifice in the right atrium and ventricle, which appeared to make it difficult to perform retrograde myocardial protection. The operative finding was consistent with the preoperative CT finding and mitral valve repair and aortic valve replacement were performed using only selective antegrade myocardial protection. We should bear in mind that small coronary ostium exists and preoperative assessment of the size of coronary sinus might be important.