Background: CRT resynchronize LV activation by pacing both RV and LV. Generally, optimal site for pacing in the LV is the latest activating segment. But due to complex pathophysiology of dyssynchrony and anatomical limitation of cardiac vein, it is often hard to predict the result of resynchronization. Optimization of CRT after implantation, adjustment of LV RV sequential pacing interval(VV timing), may further improve function of LV. But the effect of VV timing is not well established. We evaluated the effect of VV timing on LV performance. Methods: We measured ejection fraction(EF), velocity time integral(VTI) and TEI index(TEI) in the patient with dilated cardiomyopathy who implanted biventricular pacemaker, during various VV timing of LV first sequential pacing with interval 60, 40, 20, 4ms, RV first sequential pacing interval 20ms, and RV only pacing. At the same time, the degree of dyssynchrony were measured by septal lateral delay(SLD) of time to peak systolic velocity of the segment. VV timing with lowest TEI was selected as optimal timing. Result: 7 patients (M:F 5:2, age 60(35~88) were included. Optimal VV timing is LV first 60ms in 3 patients, LV first 40ms, 20ms and 4ms and RV first 20ms in 1 patient respectively. Worst VV timing with highest TEI is RV only in 3 patient, LV first 4ms in 2 patient and RV first 20ms in 1 patient. TEI index and VTI at the best and the worst VV timing were shown at table. EF and VTI is signficantly higher in the best timing than in the worst timing. Conclusion: 1. VV timing has significant effect on LV performance 2. TDI dyssynchrony index and LV performance has poor correlation during optimization. 3. CRT can be optimized considerably even after implantation, by VV timing adjustment.