Abstract Background: ST-Segment Elevation Myocardial Infarction (STEMI) is one a common presentation of cardiovascular diseases. About 40% of patients with (ACS) presented as STEMI. Prevalence of STEMI continues to rise in worldwide [1]. Platelets (PLT) have a central role in pathophysiology of ACS [2]. And a major role in development of no-reflow [6]. Furthermore, increased mean platelet volume (MPV) at ad-mission was associated with long-term mortality in patients with ACS [7]. Aim of Study: We aimed to evaluate the accuracy of MPV as a biomarker for detection of reperfusion abnormalities in STEMI patients treated with primary PCI. Patients and Methods: This study was conducted at Zagazig University Hospital on 100 consecutive STEMI patients who underwent primary PCI, blood samples obtained on admission, ECG, Echocardiography were done; those patients were divided into two groups; 80 patients who expe-rienced successful reperfusion, and 20 patients who did not. Results: Post PCI corrected TIMI frame count (CTFC) was correlated with MPV post and pre-PCI as well as MPV; (p=0.005) where patients with high mean platelet volume (MPV) had significant increase in (CTFC). MPV before and after PCI were significantly higher in unsuccessful reperfusion group compared to successful reper-fusion group. MPV showed s gnificant r duction in thethe successful reperfusion group compared to unsuccessful group (p<0.05). MPV, systolic wall motion score (SWM score) and door-to-wire time were significant and independent predictors for unsuccessful reperfusion in STEMI patients treated by primary PCI. Conclusion: Mean platelet volume estimated before, after PCI and MPV are useful biomarkers and showed high accuracy for prediction of unsuccessful reperfusion after primary PCI in STEMI patients.