Despite the success of restoration of epicardial blood flow by the primary percutaneous coronary intervention (PPCI), approximately one half of patients with ST-segment elevation myocardial infarction (STEMI) have failed myocardial reperfusion, namely microvascular dysfunction. Previous studies have shown that coronary microvascular damage significantly impacts the prognosis of patients with STEMI. Therefore, restoration of microvascular function is quite important during the PPCI procedures. Recent clinical trials found that nicorandil, a hybrid ATP-sensitive potassium channel opening agent, improved microvascular function, prevented no-reflow phenomenon, and had beneficial cardioprotective effects in patients receiving PPCI. However, there was a lack of evidence on the protective effects of nicorandil on microvascular function. The coronary angiography-derived IMR (AMR) is a novel wire-free technology developed for fast computation of IMR in patients undergoing coronary angiography, thereby enabling the quantification of coronary microcirculation of patients. This study is aimed to investigate the effects of nicorandil on microvascular dysfunction as evaluated by AMR in patients with STEMI.