Background: Ultrasound is frequently used to assess the function of heart and blood vessels. Here, we used ultrasound to evaluate the changes in vascular-graft-related parameters in patients with coronary heart disease before and after in situ left internal mammary artery (IMA) graft and assessed factors affecting blood flow in the graft. Methods: We collected data on 60 patients who underwent coronary artery bypass grafting (CABG) under the same surgeon. All ultrasonic parameters of IMA before and after CABG were measured using Philips EPIQ7 Ultrasound Machine. We then compared changes in all the parameters and assessed factors affecting IMA blood flow. Results: Postoperative IMA had wider luminal diameter than preoperative IMA, higher peak systolic and peak diastolic velocities, reduced resistance index, and significantly greater diastolic velocity-time integral and its fraction. The IMA-left subclavian artery angle was negatively correlated with intraoperative flow of vascular graft, and the blood flow of the graft was significantly higher 1 week postoperatively than it had been during the operation. Conclusions: In situ IMA bypass in CABG patients increased vascular graft luminal diameter and flow, and the angle between IMA and subclavian artery affected the postoperative flow of the vascular graft.