Abstract： Background::Various experimental and clinical studies have reported on coronary microcirculatory dysfunction ( "no-reflow" phenomenon). Nevertheless, pathogenesis and effective treatment are yet to be fully elucidated. This study aimed to measure the intracoronary pressure gradient in the no-reflow artery during emergent percutaneous coronary intervention and explore the potential mechanism of no-reflow.Methods::From September 1st, 2018 to June 30th, 2019, intracoronary pressure in acute myocardial infarction patient was continuously measured by aspiration catheter from distal to proximal segment in the Department of Coronary Care Unit, Tianjin Chest Hospital, respectively in no-reflow arteries (no-reflow group) and arteries with thrombolysis in myocardial infarction-3 flow (control group). At least 12 cardiac cycles were consecutively recorded when the catheter was pulled back. The forward systolic pressure gradient was calculated as proximal systolic pressure minus distal systolic pressure. Comparison between groups was made using the Student t test, Mann-Whitney U-test or Chi-square test, as appropriate. Results::Intracoronary pressure in 33 no-reflow group and 26 in control group were measured. The intracoronary forward systolic pressure gradient was －1.3 (－4.8, 0.7) and 3.8 (0.8, 8.8) mmHg in no-reflow group and control group ( Z = －3.989, P < 0.001), respectively, while the forward diastolic pressure gradient was －1.0 (－3.2, 0) and 4.6 (0, 16.5) mmHg in respective groups ( Z = －3.851, P < 0.001). Moreover, the intracoronary forward pressure gradient showed significant difference between that before and after nicorandil medication ( Z=－3.668, P < 0.001 in systolic pressure gradient and Z =－3.530, P < 0.001 in diastolic pressure gradient). Conclusions::No reflow during emergent coronary revascularization is significantly associated with local hemodynamic abnormalities in the coronary arteries. Intracoronary nicorandil administration at the distal segment of a coronary artery with an aspiration catheter could improve the microcirculatory dysfunction and resume normal coronary pressure gradient.Clinical trial registration::www.ClinicalTrials.gov (No. NCT 03600259).