Objectives: The aims of this study were to clarify the influence of cardiac diastolic and peripheral vascular function on the exercise capacity of patients with coronary bypass surgery (CABG) and diabetes mellitus (DM) by tissue Doppler imaging (TDI) and flow-mediated vasodilatation (FMD), and to investigate interrelations between exercise capacity and LV diastolic function, endothelial function and biochemical parameters. Methods: We analyzed the exercise capacity, TDI at the mitral annulus and FMD in 51 uncomplicated first-time CABG survivors (23 DM) at an average interval of 21.6 ± 12.2 months after surgery. Results: Diabetics had lower E’, A’, VO<sub>2</sub>peak, (a-v)O<sub>2</sub> difference, and higher E/E’ ratios (p < 0.05) than non-DM patients, but not FMD (p = 0.17). The A and E/E’ ratios correlated negatively with VO<sub>2</sub>peak after age adjustment (r = –0.336, p = 0.024). In addition, HbA<sub>1c</sub>, and triglyceride also correlated negatively with VO<sub>2</sub>peak (r = –0.377, –0.307, respectively, p < 0.05). Conclusions: Diabetics after CABG had more advanced diastolic dysfunction and oxygen extraction impairment than non-DM. It suggests these factors could contribute to lower exercise capacity, risk of developing heart failure despite preserved systolic function and poorer long-term survival of diabetic patients after CABG.