The oxidation of heavily phosphorus-doped Si(100) and polycrystalline Si in the room temperature (RT) to 800degreesC range in dry oxygen was studied by Auger electron spectroscopy (AES) and quadrupole mass spectrometry. The oxidation rate of the phosphorus-doped (P-doped) Si(100) was larger than the P-doped poly-Si at RT. In thermal oxidation at 650 degreesC in the P-doped Si(100), the delay of the dioxide. formation was found above 5 X 105 L. The AES measurements in the range of 97-137 eV showed that phosphorus oxidized in the initial stages of the oxidation of both samples. In the thermal oxidation of both samples at 800 degreesC, oxygen uptake on the silicon surfaces did not take place under 104 L. And dioxide formation took place rapidly above 104 L for both samples. Then, a large amount of phosphorus piled up at the interfaces due to the formation Of SiO2 . We gave explanations for these initial stages of oxidation of P-doped Si by the competition process between the oxidation of silicon and the desorption of SiO and P2O5.