Objective:To evaluate the impact of left or right bundle branch block(LBBB or RBBB) on LV dyssynchrony and function using phase analysis of gated myocardial perfusion SPECT. Methods: Thirty patients with LBBB, thirty with RBBB and thirty normal subjects with normal cardiac function were enrolled in this study. They all underwent gated myocardial perfusion SPECT imaging. End diastolic volume (EDV), end systolic volume (ESV) and LV ejection fraction (LVEF) were calculated from Philips Cardio MD QGS; The reconstructed data were then submitted to SyncTool to measure phase standard deviation(PSD), phase histogram bandwidth(PHB). Results: All parameters studied showed statistically significant differences between the LBBB and RBBB or control groups. There are no statistically differences between RBBB and control groups. Normal values were calculated based on control group. If PSD > 12.4° is considered dyssynchrony, the rate of LV dyssynchrony in LBBB, RBBB and NL groups is 70%, 16.7%, 0.0%, respectively. If PHB > 43.7° is considered dyssynchrony, the rate of LV dyssynchrony in LBBB, RBBB and NL groups is 63.3%, 13.3%, 3.3%, respectively. Conclusion: Phase analysis of G-MPS can effectively evaluate the LV dyssynchrony caused by LBBB accurately. LBBB can lead to obvious LV dyssynchrony and dysfunction.