Moderate hypothermia (33°C) has been shown to improve defibrillation success by DC shocks compared with normothermia (37°C) and severe hypothermia (30°C) in cardiac arrest due to ventricular fibrillation/tachycardia (VF/VT), but the mechanisms are unknown. We hypothesized that moderate hypothermia may prevent spiral wave functional reentry, and we investigated the dynamics of spiral waves induced in the two-dimensional ventricular myocardium of Langendorff-perfused rabbit hearts by means of optical mapping. Moderate and severe hypothermia (33 and 30°C, respectively) caused a significant prolongation of the action potential and a significant decrease in conduction velocity under basic stimulation at 2.5 Hz. VT/VFs induced by DC shocks often self-terminated at moderate hypothermia: the duration of TV/FVs was reduced dramatically at moderate hypothermia compared with nornothermia and severe hypothermia. Spiral waves during VT at normothermia rotated around a functional line of block and were stationary, whereas those at moderate and severe hypothermia were characterized by disorganization with frequent wave breakups. Phase maps during VT/VFs at moderate hypothermia showed collision of counter-rotating phase singularities (PSs), resulting in their mutual annihilation, and exit of PSs from the anatomical boundaries. These results suggest that moderate hypothermia may facilitate self-termination of spiral waves through the decrease of their generation/extinction ratio.