Although dual antiplatelet therapy (DAPT) has been shown to improve index of microcirculatory resistance (IMR), the importance of the early DAPT administration on IMR and left ventricular function has not been clearly defined. In this study, we aimed to assess whether early DAPT administration affect IMR, epicardial flow, and left ventricular function in ST-segment elevation myocardial infarction (STEMI) patients undergoing percutaneous coronary intervention (PCI).This was a prospective non-randomized study on STEMI receiving primary PCI in a tertiary hospital. All subjects received loading dose DAPT (Aspirin + Clopidogrel) before primary PCI. Patients were then divided into 2 groups, the first group consists of patients receiving DAPT time ≤2 hours and the second group consists of those with DAPT time >2 hours. The primary endpoint of this study was IMR, a microvasculature function index measured quantitatively by pressure-/temperature-tipped guidewire after balloon dilatation. The secondary endpoint was the mean difference of global longitudinal strain (GLS) change at 6 months follow-up, TIMI flow before, and after PCI between the 2 groups.There were 40 subjects qualified for the study, 20 subjects in each group. There was no significant difference in IMR (50.90 [34.66] vs 58.06 [45.56], P = .579) between the 2 groups. Early administration of DAPT improved ventricular function at 6 months, reflected by statistically significant greater improvement in terms of ΔGLS (-3.48 [2.61] vs -1.23 [2.87], P = .013) and Δejection fraction (10.65% [8.74] vs -0.75% [12.83], P = .002) in the DAPT time ≤2 hours group compared with DAPT time >2 hours group. TIMI flow before PCI (P = .653) and TIMI flow after PCI (P = .205) were similar in the 2 groups.Early DAPT administration ≤2 hours may improve left ventricular function, but not IMR and TIMI flow.