The initial stage of the oxynitride on Si(100) 2x1 surface by the thermal annealing incorporated with the nitrogen ion was investigated using photoemission spectroscopy (PES). After growing about 20Å thermal SiO 2 layer on Si(100) 2x1 surface, we implanted nitrogen ion with 200eV ion energy on the surface at RT. Oxynitride and Si 3 N 4 are found to co-exist during the thermal nitridation. PES results of Si 2p and N 1s were shown that N(-Si 2 O) was a major component at 800 o C annealing. The oxygen of this oxynitride was dissolved as a volatile SiO as increasing the nitridation temperature more than 1000 o C and Si 3 N 4 became a majority components. Also, N 1s core-level spectra were shifted toward the low binding energy.