The occurrence of deteriorated coronary flow associated with distal embolization during percutaneous coronary intervention results in impaired myocardial perfusion and worsens the clinical prognosis. This study aimed to examine the impact of optical coherence tomography (OCT)-determined coronary plaque morphology on the prediction of deteriorated coronary flow after stent implantation in acute as well as stable coronary syndromes (ACS and SAP, respectively). We studied 126 patients who underwent OCT during stenting for ACS (n = 44) and SAP (n = 82) with a de novo lesion. Angiographic deteriorated coronary flow was defined as the deterioration of TIMI flow grade after mechanical dilatation in the absence of a mechanical obstruction on angiograms. Patients could be divided into the deteriorated flow group (n = 21) and the reflow group (n = 105). Under these conditions, the presence of thin-cap fibroatheroma (TCFA) was more frequently observed in the deteriorated flow group than in the reflow group in both ACS and SAP. A multivariable logistic regression model revealed that TCFA was an independent predictor of deteriorated coronary flow (hazard ratio: 12.32; 95 % confidence interval: 3.02–50.31; p = 0.0005). These results demonstrate that TCFA detected by OCT could be a strong predictor of the occurrence of deteriorated coronary flow during stent implantation in ACS as well as SAP.