Background: Resting heart rate (HR) is a strong predictor of cardiovascular mortality and morbidity in patientswith heart failure with reduced ejection fraction (HFrEF). However, the effects of HR-lowering therapy on diastolic function in HFrEF patients are notwell described. In this study,we aimed to investigate the effect of lowering HR on diastolic function in HFrEF patients with sinus rhythm. Methods: Fifty patientswith HFrEF with coexisting diastolic dysfunction and sinus rhythm with resting HR > 70 bpm were prospectively included in the study. All patients were treated with intended HR-lowering therapy, which targeted a HR below70 bpm.We divided thewhole population according to the resting HR achieved with strict rate control (group 1) and to that achieved without strict rate control (group 2; HR > 70 bpm) at the end of the study. Left ventricular diastolic function and B-type natriuretic peptide (BNP) values at baseline and at the end of the study were compared in both groups. Results: No significant differences were found between the groups in terms of baseline parameters except for lower diastolic blood pressure in group 2. At the end of follow-up, E/Em ratio, E/A ratio and left atrial area significantly decreased with an increased deceleration time in group 1. The changes in HR (delta HR) were correlated with E/Em (r = 0.67, p < 0.001) and delta BNP level (r = 0.49, p < 0.001). Conclusions: This study showed that an effective HR lowering in patients with HFrEF can lead to improvements in diastolic function.