We control the ion density ratio of [N+]/[N2+] with the voltage-biased grid system in inductively coupled nitration plasma. The ion density ratio is controlled from 0.39 to 0.04 with decreasing grid-biased voltage. We try to analyze the variation of the ion density ratio using the measured plasma parameters and particle balance equation. The important factor determining the ion ratio is the plasma potential difference between the source region—where plasma is generated—and the diffusion region—where the electron temperature is controlled. When the plasma potential is higher in the source region than in diffusion region, the ion density ratio is determined by the electron temperature in Region I. Inversely, the ion density ratio is determined by the electron temperature in Region II, when the plasma potential is higher in Region II than in Region I.