Background: Coronary artery spasm is an important cause for anginal chest pain, but it is not appropriately estimated, in patients with angiographically non - obstructive coronary arteries. In acute coronary syndrome, coronary artery spasm is the cause of 49% of cases and appears in 30% of cases without the culprit artery. In addition, the elimination of coronary spasm on the narrow segment and the non-narrow segment helps to more accurately assess the degree of stenosis (%) of the lesion. So we implemened the study to find out the variation of coronary artery diameterbefore and after intra coronary nitrate injection. Methods: A cross - sectional descriptive study was conducted on 40 patients diagnosed ischemic heart disease Results: The minimum diameter of the lumen artery significantly increased from 1.25 ± 0.35 mm to 1.56 ± 0.47 mm. Similarly, the area of the smallest lumen artery also increased from 1.32 ± 0.74 mm2 to 2.04 ± 1.26 mm2. The parameters of the coronary arterial lumen reference also increased significantly, from 2.84 ± 0.65 mm to 3.02 ± 0.70 mm for the diameter, and from 6.68 ± 3.05 mm2 to 7.56 ± 3.39 mm2 for area. Despite the dilation of the smallest lumen artery and the reference one, we recorded the diameter of stenosis (%) decreased from 55.85 ± 6.77% to 48.75 ± 9.13%. It is worth noting that there were more than 50% of cases showing the significant stenosis of diameter decreased to less than 50% of stenosis (in the stenosis group of no significance). Conclusion: Intracoronary nitrate injection in coronary artery helps to accurately assess the stenosis and the diameter of the coronary artery.