Background We hypothesize that spatiotemporally controlled shocks (STCS) on the activation of the low voltage gradient area (LVG) can improve defibrillation efficacy. Using a real-time synchronized pacing (Sync P) technique, we synchronized the ventricular fibrillation (VF) and then delivered synchronized shocks (Sync Shock) when the low voltage gradient (LVG) area was in the refractory period (RP). Method and Results We explored STCS, which was composed of 0.92 sec of Sync P at the low voltage gradient area followed by a Sync Shock in 13 isolated rabbit hearts. For Sync P, optical action potentials (OAPs) adjacent to four pacing electrodes (10 mm apart) were monitored. When one of the electrodes was in the excitable gap, a 5mA current was administered from all electrodes. Sync Shock was delivered 23ms after the excitable gap. STCS was compared with random shock (C) for DFT50 (n=35 for each) and shock timings by 25-point OAP analysis (n=208 for STCS, n=172 for C). The results showed that 1) Sync P caused wavefront synchronization as indicated by a decreased number of phase singularity points (9.1짹3.1 to 5.4짹2.6, p<0.0001) and reduced spatial dispersion of VF cycle length (6.8짹4.2 ms to 4.7짹3.5 ms, p<0.01). 2) STCS decreased DFT50 by 10.3% (174.3짹75.7 V to 154.0짹61.4 V, p<0.05). 3) The successful shocks showed shorter pre-shock coupling intervals (CI; 37.0짹24.1 ms vs. 49.7짹25.7 ms, p<0.05) and a higher proportion of RP shock (p<0.001) than failed shocks. STCS showed shorter CIs (38.0짹23.2 ms vs. 48.5짹27.4 ms, p<0.05) and a higher RP shock rate (p<0.05) than C. Conclusion Spatiotemporally controlled defibrillation shock delivers the shock during refractoriness in the LVG area, resulting in improvement of defibrillation efficacy.