OBJECTIVE: Systemic immune-inflammation index (SII), on the basis of lymphocyte, neutrophil and platelet counts had been published to be a good prognostic factor in coronary artery disease. Nevertheless, the prognostic value of Systemic immune-inflammation index (SII) in a condition of no-reflow phenomenon (NRP) remains inconsistent, we evaluated the SII as a simple calculated tool for predicting the NRP among patients with STEMI who underwent primary percutaneus coronary intervention (PCI). METHOD: 510 consecutive acute STEMI patients who underwent primary PCI within 12 h from symptom onset from October 2015 to January 2020 were enrolled in our study. The receiver-operating characteristic (ROC) curve was used to determine the cut-off value of SII to predict the no-reflow. Multivariate stepwise logistic regression, including covariates found to have a significant association with NRP in univariate analysis, was used to identify independent predictors of no-reflow. RESULTS: A ROC curve analysis showed that the best cut-off value of SII for predicting no-reflow was 1028, with sensitivity and specificity of 79% and 70, respectively (AUC, 0.839; 95% CI 0.797-0.881). An ROC curve comparison analysis was performed to compare the SII with NLR and PLR. Multivariate analysis revealed that SII ≥1028 value (OR = 6.622, 95% confidence interval (CI): 3.802-11.627,  < .001), not using aspirin prior to admission (OR = 0.431, 95%CI: 0.236-0.786,  = .006), and CRP (OR = 1.004, 95%CI: 1.001-1.008,  = .041) were independent predictors related to occurrence of NRP after primary PCI in patients with acute STEMI. CONCLUSION: SII levels are independently associated with the NRP in patients undergoing primary PCI for acute STEMI. High SII may be a promising indicator for the prediction of NRP in these patients.