By using scanning tunneling microscopy and spectroscopy (STM and STS), the initial stages of NH3 exposure on Si(111)-(7x7) at different substrate temperatures and dosages have been studied. At room and very high (similar to1050degreesC) temperatures, the 7x7 surface structure remains and the nitrided sites appear darker, randomly distributing on the surface. Moreover, we find a constant ratio (similar to3.46-3.83) of reacted center adatoms to reacted corner adatoms on the partially nitrided surfaces. At intermediately temperatures (similar to900degreesC), the majority (>90%) of the reacted surface forms the well-ordered silicon nitride 8 X 8 reconstruction. In this regime, hexagonal- and triangular-shaped nitride islands can be observed on the 8x8 and 7x7 surfaces, respectively. We have also used STS to investigate the changes of local density of states on the nitrogen-reacted 7x7 surfaces prepared by different conditions.