Aim: To investigate the effect of defective barrier on the optimum breakdown voltage, using positive and negative needle electrodes in an air medium of 10cm gap distance. Methodology: The barriers for the tests were placed at 2.5cm from the point electrode for each test. The defective barriers were created by having holes of 6mm, 8mm, 12mm and 20mm diameter at the centre of the barrier. For each barrier position the breakdown test for positive and negative polarity for needle electrodes were carried out. Also, tests were carried out with non defective barrier and with point-plane airgap (without barrier). Result: From the test without barrier the negative point electrode offered higher breakdown voltage (1.8 times), than the positive point. When with plain barrier the positive point was optimized to 1.6 times, while the negative point was lowered. The optimum breakdown voltage decreased gradually as the hole diameter increased and at 20mm hole diameter the effect was like the plain barrier. Conclusion: From the results, optimization is only effective with positive point’s electrode and it endures even with small opening within the ionization zone. It is necessary to check this in practical situations because the specified optimum voltage of an equipment may be lowered.