Background: Based on the studies which have been done till now, resynchronization of ventricular contraction results in clinical improvement in patients who suffer from moderate to severe heart failure with intraventricular conduction disturbance. and using this method is increasing in our country. Study in this filed seems to be necessary.Material and methods: Sixty-five consecutive patients with advanced heart failure, NYHA Class III and IV, LV ejection fraction (EF)£35%, QRS³120 were studied before and 3 months after CRT. Mean age was 60, 57 and 8 patients with NYHA class III and IV respectively. All patients received furosemide, spironolactone, carvedilol, ACE inhibitors and angiotensin II –receptors blockers at the optimal tolerated dosage. Mean LVEF was 20.83%. Echocardiographic (Tissue Doppler Imaging), ECG and Function class (NYHA) were assessed at baseline before implantation and 3 months after.Result: LV ejection fraction increased +6.8% (21.10±7 vs. 27.93±8.4%, P<0.001). Mitral valve regurgitation severity reduced (P<0.013). Interventricular mechanical delay reduced (53.83ms±18.45 vs. 26.53ms±13.90 P=0.000) and also Intra ventricular mechanical delay reduced (62.53ms±25.08 vs. 45.55ms±13.47, P=0.000). Dilated cardiomyopathic group has more mean changes in LVEF than Ischemic group (+9.06%±7.3 vs. +4.08%±6.5, P=0.01).Conclusion: Biventricular pacing results in significant clinical improvement in patients who have moderate to severe heart failure.