Electric shock can terminate reentrant wave or fibrillation in ventricle. The goal of the present study is to examine the effect of shock strength on defibrillation efficiency. For this purpose, we developed a computational model of virtual defibrillation based on a realistic 3D geometry of canine ventricles. The ventricles were electrically paced by virtual electrodes, and then truncated-exponential monophasic and biphasic shocks with varying shock strength are applied via external electrodes. This study used bidomain method to simulate electric wave phenomena in ventricles, and a finite element method with Galerkin approximation is used to discretize the 3D domain spatially. We applied effective strength of monophasic and biphasic shocks to the ventricular fibrillation model and observed whether the fibrillation is terminated or not by the specific strength of the imposed electric shock.