A low‐temperature plasma oxidation of GaAs (lower than 100 °C) has been realized. The oxidation apparatus mainly consists of a quartz tube chamber, a low‐power rf oscillator and an electrical magnet. The oxidation rate can be controlled in the range 100–600 A˚/min by changing the magnetic field perpendicularly applied to the sample. The interface state density betweenp‐type GaAs and its oxide film is the order of 1010cm−2 eV−1around 0.5 eV from the top of the valence band. This low state density suggests that the oxide film can be applied to various GaAs MOS devices. For the oxide film ofn‐type GaAs, an anomalous frequency dispersion in the MOS capacitance is found in the accumulation region. This anomaly is very similar to that observed in anodic oxidation.