The effects of artificial obstacles on the dynamics of ventricular fibrillation have been extensively investigated with an electrical mapping system. This study was performed to assess the influence of transmural obstacles on the dynamics of wavefronts and to determine whether obstacles can convert ventricular fibrillation to ventricular tachycardia by stabilizing wavefronts in fibrillating right ventricular tissues of pigs using an optical mapping system. The right ventricles of pigs(n=15) were excised and placed in a tissue perfusion system with the epicardium facing up. A hole of increasing size from 2 mm to 8 mm in diameter was created using a skin biopsy punch. Then, the other 8mm sized hole was made just beside the first hole. The changes of wavefront dynamics and the cycle length of optical action potential waves were investigated. In 14 among 20 obstacles of ten tissues, transient attachment of electrical activities along the rim of obstacles and transient rotation of wavefronts were observed. During baseline ventricular fibrillation, fibrillation cycle length was 118.5±24.7 msec and this was increased to 135.4±30.2 msec after 8mm hole, and to 159.4±47.7 msec after 2 holes(p=0.01). There was a positive correlation between the obstacle size and cycle length(r=0.43, p=0.007). In three tissues, conversion to ventricular tachycardia from ventricular fibrillation was observed after creation of two holes. Obstacles of proper size had anti-fibrillatory effects in tissues with ventricular fibrillation and this phenomenon was partly explained by the temporary attachment of wavefronts to the obstacles.