Abstract Objectives The acute phase of ST-segment elevation myocardial infarction (STEMI), as determined by TIMI angiographic criteria, is influenced by various factors that impact the patient’s clinical outcome. However, the modifiable risk factors of impaired TIMI flow (TIMI<3) and its effective treatment are not fully understood. Hyperglycemia may induce a pro thrombotic state and thus affect TIMI flow before or after PCI. This study investigates the correlation between hemoglobin A1c levels, TIMI flow grade, and thrombus grade in infarct-related arteries, assessing its predictive value in non-diabetic patients with STEMI. Methods The 265 patients selected based on the hemoglobin A1c level lower than 6.5 % and were divided into three groups based on HbA1c level. Comparison between three groups in terms of risk factors, troponin level, blood glucose level, lipid profile, kidney function, number of involved vessels, type of MI, left ventricular ejection fraction, TIMI flow before and after primary angioplasty, thrombus burden, complications and hospital mortality was made. Results With the increase in HbA1c level, the prevalence of TIMI 3 flow after primary PCI decreased. The prevalence of TIMI flow 2–3 before angioplasty also decreased with the increase in HbA1c level. Increased hemoglobin A1c was also significantly related to large thrombus burden (p=0.021). Morover, hemoglobin A1c remained an independent predictor of post-PCI TIMI flow and thrombus burden. Conclusions Elevated hemoglobin A1c is a predictor of TIMI flow less than 3 after primary PCI and high thrombus burden, in STEMI patients without a history of diabetes mellitus.