Background and objective Thrombus embolization during primary percutaneous coronary intervention (PPCI) is the main cause of microvascular obstruction. Effective manual thrombus aspiration may enhance myocardial perfusion and reduce infarct size. The aim of study is to assess the effect of manual thrombus aspiration in ST-segment elevation myocardial infarction (STEMI) during PPCI, which assessed by cardiac magnetic resonance (CMR) imaging. Methods STEMI patients who were performed CMR after PPCI during hospitalization were analyzed retrospectively. Manual thrombus aspiration was done by using Thrombuster II catheter (Kaneka Corp. Japan). Repeated aspiration to minimize residual thrombus was allowed. The primary endpoints were TIMI myocardial perfusion (TMP) grade and infarct size by CMR. Results Eligible patients for this study were twenty-eight (mean age 55 짹 9.5 years, 26 males). There were no significant differences in symptom-to-balloon time (238.9 짹 142.4 vs. 320.7 짹 201.1 minutes, p=0.225), ejection fraction (51.9 짹 3.7 vs. 51.0 짹 10.6 %, p=0.770) and peak troponin I levels (47.2 짹 10.5 vs. 48.0 짹 7.6 ng/mL, p=0.828). Thrombus aspiration (TA) group (14 patients) showed significant higher TMP grade compared with no thrombus aspiration (NA) group (14 patients) (2.5 짹 0.7 vs. 1.9 짹 0.7, p=0.038). Moreover, Infarct size was significantly reduced in TA group (16.8 짹 8.9 vs. 26.1 짹 11.4 %, p=0.023). No differences were observed in the presence of microvascular obstruction and transmural infarction (64.3 vs. 85.7 %, p=0.385; and 42.9 vs. 57.1%, p=0.706). Conclusion Manual thrombus aspiration enhances myocardial perfusion and reduces infarct size during PPCI.