Heart failure (HF) is one of the main causes for cardiovascular morbidity and mortality. This study was designed to examine the effect of PDE-5 inhibition on cardiac geometry, function and apoptosis in post-infarct HF. Our data revealed that treatment of the PDE-5 inhibitor sildenafil, beginning 3 days after left anterior descending coronary artery ligation, attenuated LV remodeling, cardiac dysfunction, cardiomyocyte apoptosis and mitochondrial anomalies including ATP production, mitochondrial respiratory defects, decline of mitochondrial membrane potential (MMP) and compromised mitochondrial ultrastructure. Sildenafil partially ameliorated the downregulation of Sirt3 protein and acetylation of PGC-1alpha in peri-infarct myocardial regions. In cultured neonatal mouse ventricular myocytes subjected to hypoxia for 24 hrs, sildenafil suppressed apoptosis, promoted ATP production and elevated MMP, along with the increased Sirt3 protein expression and decreased PGC-1alpha acetylation. Interestingly, knock down of Sirt3 attenuated or nullified sildenafil-offered beneficial effects. Our findings demonstrated that sildenafil exerts its cardioprotective effect against post-infarction injury by improving mitochondrial ultrastructure and function via the Sirt3/PGC-1alpha pathway. This observation should shed some lights towards application of sildenafil in energy-related cardiovascular diseases.