Objective To determine the level of serum endothelial nitric oxide synthase (eNOS) in patients with coronary slow flow (CSF) and evaluate its correlation with CSF. Methods Thirty-six patients with CSF diagnosed by coronary angiography in our hospital from Jan. 2018 to Dec. 2019 were enrolled (CSF group), and 36 patients without CSF phenomenon who underwent coronary angiography during the same period were enrolled as control group. The coronary flow velocity was evaluated by thrombolysis in myocardial infarction frame count (TFC), serum eNOS level was measured by enzyme-linked immunosorbent assay, and the correlation between them were analyzed. Results TFC and mean TFC (mTFC) of all coronary arteries (left anterior descending, left circumflex, and right coronary artery) in the CSF group were significantly higher than those in the control group (all P<0.001), and the serum eNOS level in the CSF group was significantly lower than that in the control group ([46.40±5.57] ng/L vs[59.91±8.75] ng/L, P<0.001). Pearson correlation analysis showed that there was a negative correlation between mTFC and serum eNOS level (r=-0.658, P<0.001). Conclusion Serum eNOS level of CSF patients is significantly decreased, and it is negatively correlated with coronary flow velocity. Low eNOS expression may be an important cause of CSF.