Plasma basic ion implantation (PBII) with high negative voltage pulses has been applied to test specimens in a sterilization process as a technique suitable for three-dimensional workpieces. Pulsed high negative voltage was applied to the electrode in this process at the gas pressure of oxygen. It was reported that the PBII process reduced the numbers of active Bacillus pumilus cells using self-ignited plasma N2 gas generated by only pulsed voltages. The number of bacteria survivors was reduced by 10-5with a few min exposure. As the ion energy is the most important processing parameter, a simple method to estimate the oxygen ion energy calculated using distribution for oxygen in Si implanted by PBII was estimated. In this work, the Effect of Plasma Density on Residual Bacterial Number and Applied Voltage is studied.