Background Dilated cardiomyopathy (DCM) is a primary cardiomyopathy characterized by the enlargement of left ventricle or biventricular and left ventricular systolic dysfunction, without any obvious stenosis of coronary arteries. However, it remains unclear that whether the diameter of coronary artery is different from those of normal population, because few studies directly assessed the diameter of the coronary artery in patients with DCM. The study aimed to evaluate the diameter of coronary arteries in DCM patients and its influence on prognosis for DCM patients. Methods Fifty-seven DCM patients and fifty-nine patients presenting with atypical chest pain and normal coronary angiography were enrolled in the study. A coronary angiography image analysis system was used to measure the diameters at 5 mm from the opening of the left main coronary artery (LM), left anterior descending (LAD) and left circumflex coronary (LCX) arteries, and the right coronary artery (RCA) of both groups. Follow-up study through telephone over a period of 2 years was performed. Spearman rank correlation and logistic regression were used to evaluate the correlations of the diameter of coronary arteries with the risk of nonfatal heart failure event. Results Diameters of the LM, LAD, LCX, and RCA in DCM group were significantly larger than those of the control group (P ＜ 0.001). During a follow-up of 2 years, the nonfatal heart failure event occurred in 9 patients of DCM group, but not in control group. Spearman rank correlation analysis showed diameters of the LM, LAD, and LCX were correlated with the risk of nonfatal heart failure event, respectively (P ＜ 0.05). While the diameter of RCA showed no correlation with the risk of nonfatal heart failure event (P = 0.583). Whereas logistic regression analysis showed there were no correlation between diameters of the LM, LAD, LCX, and RCA and the risk of nonfatal heart failure event (P ＞ 0.05). Conclusions The coronary arteries of DCM patients show a larger diameter without any obvious stenosis, which may not correlate with the risk of heart failure event.