Atomic-order nitridation of Si(100) in an NH3 environment (124-1400 Pa) at 300-650 degrees C has been investigated using an ultraclean low pressure hot-wall reactor system. At 500 degrees C or higher, the N atom concentration (n(N)) initially increases and tends to saturate to a certain value (d similar to 5 Angstrom, n(N) similar to 3 x 10(15) cm(-2)). At 400 degrees C or lower, on the H-terminated Si surface, the Si-hydride decreases with increasing NH3 exposure time and becomes hardly observed when n(N) reaches nearly to the surface Si atom concentration (6.8 x 10(14) cm(-2)). On the H-free Si surface, n(N) increases up to similar to 2 x 10(14) cm(-2) with the appearance of the Si-hydride instantly after NH3 exposure. It is expected that NH3 dissociatively adsorbs on the Si dangling bonds. It is found that n(N), is well described by Langmuir-type physical adsorption and reaction of NH3 on the Si surface. The ultrathin nitride film shows Very good characteristics as a mask against oxidation.