A 6-year-old boy with left main trunk (LMT) stenosis, who had undergone arterial switch operation (ASO) for transposition of the great arteries( TGA) before 6 years, underwent LMT orifice reconstruction. Coronary angiography showed severe stenosis of LMT, already when he was hospitalized with heart failure after 3 months of ASO. He was stable with oral treatment, therefore we performed the LMT reconstruction, before starting school, in terms of the risk of coronary ischemic event. We reconstructed the LMT ostium using an aortic flap and autologous pulmonary arterial patch. The postoperative computed tomography showed neither stenosis nor kinking at the repair site of LMT. He remains asymptomatic for over 6 months. We have reported that this method is effective to repair coronary artery anomalies (CAAs), since using an aortic flap. We could form a coronary artery floor, which is affixed to the aortic wall. This method allows us to change the coronary orifice position and the angle, so it is very useful method not for only CAAs, but also for coronary stenosis after ASO.