Objective To analyze the electrocardiographic（ECG） characteristics for the first diagonal branch of infarction relatedartery（IRA） in patients with acute ST-segment elevation myocardial infarction（STEMI）. We observed the incidence rate of South African Flag Sign

in order to help the physician to make a quick diagnosis.Methods A total of 19 STEMI patients with coronary angiography（CAG） confirmed first diagonal branch of IRA were retrospectively analyzed. The patients were treated in our hospital from January 2016 to December 2020 and their ECG changes at admission were studied for ST-segment elevation/depression changes at different leads in all patients.Results Eighteen（94.7%） patients had ST-segment elevation in lead I and a VL. The incidence of ST-segment elevation in the chest lead was

in turn as 12（63.20%） patients in lead V2

2（10.5%） in each lead V3and lead V4

1（5.3%） in lead V1

lead V5and lead V6; while ST-segment depression was as 19（100%） patients in lead III

15（78.9%） in lead a VF

12（63.2%） in lead II and 1（5.3%） in lead a VL respectively. The probability of South African Flag Sign was 63.2%. Conclusions The incidence of South African Flag Sign in the first diagonal branch of infarction relatedartery（IRA） in patients with acute ST-segment elevation myocardial infarction（STEMI） was relatively high

which provides a reliable basis for doctors to improve their diagnostic accuracy.