Fractional flow reserve (FFR) is considered as the gold standard for physiological assessment of coronary artery stenosis. However, it may be difficult to interpret FFR for the stenosis of the donor artery of chronic total occlusion (CTO), because revascularization of CTO may improve FFR of the donor artery. We present a case of 32-year-old male who had a CTO in right coronary artery (RCA), 90% stenoses in left circumflex artery (LCx) and a mild stenosis in the middle segment of left anterior descending artery (LAD). FFR for the mild stenosis in LAD showed significant value (0.72). However LAD was the donor artery to CTO of RCA, revascularization to RCA was expected to improve FFR for LAD. As the patient had chronic granulocytic leukemia and the difficulty in continuing dual antiplatelet therapy, we selected coronary artery bypass grafting (CABG) to RCA and LCx, and we decided not to perform anastomosis to LAD. Although each graft was patent and collateral flow from LAD to RCA disappeared after CABG, FFR for LAD was still 0.72. Careful consideration should be given when interpreting FFR for the donor artery to a CTO lesion. When CABG is selected, it may be a practical approach to revascularize not only CTO but also FFR positive mild stenosis simultaneously, even though it appears angiographically mild stenosis.