Introduction: Primary PCI for STEMI in lesions with a occlusive thrombus load increases the procedural complication rate and remains a challenge. But distal protection devices have no clinical benefits despite of high cost and operator's labor. Hypothesis: We assessed the hypothesis that active manual thrombosuction using export aspiration catheter before actual primary PCI procedure may effective method for improve the myocardial reperfusion and reduce the microembolization. Methods: We randomly assigned 143 patients with STEMI who were scheduled to primary PCI to initial suction group or actual primary PCI group. Thrombosuction was done unless cardiogenic shock, LMCA disease, TIMI 3 with no visible thrombus after guidewire passage or reference diameter< 2 mm. The primary end points were post-procedural myocardial blush grade (MBG), rates of no reflow and ST resolution ���50%.Results : 133 patients (60.8 짹 1.05 years, male 69.9%) entered the analyses. The culprit vessels were LAD 42.9%, LCX 6.7% and RCA 52.3%, most lesions with proximal location(51.8%). The thrombus scores were higher in suction group, the rates of post-procedural MBG 2 and 3 were 59.7% and 23.9% in the suction group compared with 56.1% and 12.1% in the actual primary PCI group respectively (p=.05). The rates of no reflow, distal embolization and ST resolution ���50% were higher in suction group than actual primary PCI group without significance. Conclusions:This prospective randomized study showed that simple manual thrombosuction in patients with STEMI before actual primary PCI results in better angiographic myocardial reperfusion rates despite of initial high thrombus burden. In conclusion, primary thrombosuction technique may effective method for improve the myocardial reperfusion and thrombus burden reduction in patients with STEMI.