Aim To investigate the relationship between intron 4a/b gene polymorphisms of endothelial nitric oxide synthase (eNOS) gene and coronary artery ectasia (CAE). Methods The research was performed by case-control study. The CAE group included 30 patients with coronary artery ectasia on coronary angiogram. The control group contained 41 patients with normal coronary artery. The study collected the data of patients'gender, age, smoking history, drinking history, ect. Polymorphism chain reaction (PCR) technique was used to identify the eNOS intron 4a/b gene polymorphisms. Results The lesion involved single coronary artery in most cases was 46.7%(14 cases), the lesions involved in two and three vessels were 26.7%(8 cases)and 26.7%(8 cases), respectively. The right coronary artery (RCA) was the most frequently involved vessel (44.4%), the left anterior descending artery (LAD) and the left circumflex artery (LCX) were 31.5% and 24.1%, respectively. The frequencies of eNOS gene phenotypes in CAE group and control group for “aa”, “ab”, “bb” were 13.3%, 33.3%, 53.3% and 4.9%, 17.1%, 78%(P＞0.05), respectively. The presence of “a” type allele of eNOS gene in CAE group and control group were 30% and 13.4%(P<0.05), respectively. Logistic regression analysis showed that “a” type allele of eNOS gene was an independent risk factor for CAE (P<0.05, OR =3.327, 95% CI=1.083 ~ 10.226). Conclusion The “a” type allele of eNOS gene may be an independent risk factor for the occurrence of CAE.