Background: Although transradial coronary artery angiography (TRCAG) has been developed to minimize complications and improve the quality of life, it could give the harmful effects on radial artery (RA) as a conduit for coronary artery bypass graft surgery (CABG). The purpose of this study was to evaluate the effects of trimetazidine (TMZ) on endothelial functional recovery of RA in patients undergoing TRCAG by using flow-mediated dilation (FMD). Methods: Sixty patients, who were underwent TRCAG with 5Fr catheter, were randomly assigned to TMZ or control group. The baseline, post-CAG (within 24 hours after TRCAG), and 10-week follow-up FMD were performed. Results: In non-cannulated arms of both groups, there were no changes of FMD at post-CAG and 10-week follow-up compared with baseline FMD in both groups. In control group of cannulated arm, the baseline FMD was 11.4짹3.5%, but the post-CAG FMD was significantly decreased to 4.2짹2.9% (p <0.01). At 10-week follow-up, the FMD was still decreased compared with baseline FMD (11.4짹3.5 vs. 5.9짹3.3 %, p < 0.01). In TMZ group of cannulated arm, the difference of FMD between baseline and post-CAG was also significant (11.3짹3.5% vs. 6.3짹4.0%, p <0.01). But at 10-weeks follow-up the FMD was improved to 10.4짹3.4 %. Consequently, there was no difference of FMD between baseline and 10-week follow-up (11.3짹3.5% vs. 10.4짹3.4%, p=0.26). Conclusions: TMZ therapy after TRCAG improved endothelial function of RA. This may contribute to positive effect on conduit function and graft longevity after CABG.