[en] Oxygen plasma immersion ion implantation (PIII) has been conducted on AZ31B magnesium alloy using different bias voltages. The modified layer is mainly composed of MgO and some MgAl₂O₄. Results form Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) indicate that the bias voltage has a significant impact on the structure of the films. The oxygen implant fluences and the thickness of the implanted layer increase with higher bias voltages. A high bias voltage such as 60 kV leads to an unexpected increments in the oxygen-rich layer's thickness compared to those of the samples implanted at 20 kV and 40 kV. The hardness is hardly enhanced by oxygen PIII. The corrosion resistance of magnesium alloy may be improved by a proper implantation voltage. (low temperature plasma)