Thermal effects in atomic-order nitridation of Si(100) by an electron-cyclotron-resonance nitrogen plasma were investigated by controlling the Si surface temperature. It is found that the N atom concentration increases linearly and then tends to saturate with increasing plasma exposure time. in the radical reaction limited nitridation, the N atom concentration on the Si surface-is normalized by the radical density in the nitrogen plasma, and the nitridation rate does not depend on the Si surface temperature above about -25 degreesC and decreases with decreasing the temperature below about -25 degreesC. The saturated N atom concentration is a single atomic layer and more than double atomic layers on the Si surface of about - 80 degreesC and above about - 25 degreesC, respectively. On the other hand, by the contribution of the incident ion, the nitridation of the deeper Si atoms below the surface is enhanced, and the saturated N atom concentration is lower at higher pressure and lower Si surface temperature. Consequently, it is suggested that the nitridation of the deeper Si atoms below the surface is enhanced with increasing ion energy as well as the Si surface temperature.