This study aimed to test the hypothesis that left ventricular dyssynchrony may negatively affect left atrial (LA) dyssynchrony and reservoir function, and cardiac resynchronization therapy (CRT) may improve LA function. It also assessed, whether residual LA dyssynchrony affects the prognosis in patients with heart failure with reduced ejection fraction (HFrEF). Ninety subjects were included: 40 HFrEF patients with a wide-QRS complex (≧130 ms), 28 HFrEF patients with a narrow-QRS, and 22 normal controls. LA global longitudinal strain (LA-GLS) and LA dyssynchrony were quantified by speckle-tracking strain analysis. LA dyssynchrony was defined as the maximal difference of time-to-peak strain (LA time-diff). All patients with a wide-QRS underwent CRT, and event-free survival was tracked for 24 months. At baseline, LA dyssynchrony was significantly more pronounced in patients with a wide-QRS HFrEF (342 ± 126 ms) than that in patients with a narrow-QRS (236 ± 127 ms, P < 0.001) and controls (186 ± 78 ms, P < 0.001). Six months after CRT, LA-GLS significantly improved from 11.9 ± 4.7 to 19.6 ± 10.1% (P < 0.05) and LA time-diff was reduced from 338 ± 123 to 245 ± 141 ms (P < 0.05) in responders only. Patients with an LA time-diff < 202 ms and those with an LA-GLS ≧14.6% six months after CRT showed significantly better outcomes than the others (P < 0.05, respectively). Among the responders, those with an LA time-diff < 202 ms after CRT showed a better prognosis than others (P < 0.05). CRT improved LA dyssynchrony and reservoir function through the improved left ventricular coordination. Reduced LA dyssynchrony and improved LA reservoir function after CRT lead to better outcomes.