Background: The angiotensin receptor-neprilysin inhibitor sacubitril/valsartan is known to improve outcomes of cardiac death and hospitalization due to heart failure in patients with heart failure and reduced ejection fraction (HFrEF). However, data on improvements in ejection fraction after using sacubitril/valsartan are still lacking in Taiwan. Methods: We conducted this prospective, single armed, observation cohort study to evaluate changes in left ventricular ejection fraction (LVEF) in patients with heart failure and reduced LVEF treated with sacubitril/valsartan. This was an all-comer study.We prescribed sacubitril/valsartan as both first-line and second-line therapy to every eligible patient regardless of whether they were already on standard therapy or newly-diagnosed with HFrEF. The primary outcomewas improvements in LVEF.We also collected data about changes in left ventricular chamber size, blood pressure, N-terminal pro-B-type natriuretic peptide (NT-proBNP), and renal function according to serum creatinine level. Results: During March 2016 to April 2018, 93 patientswere enrolled. The mean LVEF improved from 35 6.1% to 50 8.8% at 6 months use of sacubitril/valsartan (p < 0.001). The left ventricular end-diastolic diameter, left ventricular end-systolic diameter, and left atrial diameter all decreased. The average NT-proBNP level decreased from 6379 pg/mL to 1661 pg/dL. Conclusions: Sacubitril/valsartan demonstrated a significant effect in improving LVEF, left ventricular reverse remodeling, and reduction of NT-proBNP in this Taiwanese cohort.