Background & Objective : Heart failure is a clinical syndrome and increases in rate by aging. Because changes in ECG and electrolytes are variable, specific and sometimes independent to the disease with different methods to be diagnosed, in this study we tried to find a way to diagnose them.   Materials & Methods: This is a retrospective descriptive-analytic study. Data was collected from decompensated heart failure patients with EF<40% admitted to one of the educational hospitals in Tehran from 1382-84 with analyzing ECG changes (at least 3 times), cardiac enzyme, blood glucose and serum electrolyte in their files. Patients with severe valvular heart disease were excluded from this study data was analyzed by SPSS 11.5.   Results: 157 patients studied, mean age: 69.69 ± 10 years, 54.8% men, 45.2% women, median of EF range was 30 ejection fractions was 30. 42.5% had normal heart rate (60-100). The changes were: Sinus tachycardia in 30.6%, abnormal (low) heart rate variation was present in 93.1% with no relation with EF. increased QT interval in 77.7% that had significant relation with EF (p<0.05), left axis deviation in 30.6%, left atrial enlargement in 58%, left ventricular hypertrophia in 50.3%, none of them had any relation with EF 7% had low voltage that had significant relation with EF. Poor-R progression in 25.5% and Q wave in 35% both had no relation with EF. In 79%, ST.T had changes with significant relation with EF (p<0.009). First degree A-V blocks in 26.8%, second in 1.3% & AF rhythm in 16.6% that all had no significant relation with EF. Premature ventricular contraction (PVC) in 20.4%, that had significant relation with EF (p<0.008), left bundle branch block in 15.9% that had significant relation with EF (p<0.001). LDH in 59.1%, CPK in 9% and CK-MB in 19.3% were increased. 44.2% had abnormal fasting blood glucose at the time of admission. Abnormal sodium level in 20.6% and abnormal potassium level in 15.7% was present.   Conclusion: In a vast majority of patients in this study, we detected different ECG changes such as atrial and ventricular hypertrophia, ST-T change, sinus tachycardia and … some according to the severity of heart failure, some not. So it seems that paying attention to the ECG changes is helpful in diagnosing & prognosis assessment of patients with heart failure.