Silicon nitride thin films were deposited by rf magnetron sputtering with various gas flow conditions (N-2/Ar). The effect of gas flow conditions on the structure and compositions of the films was investigated. X-ray photoelectron spectroscopy indicated that the binding energy of the Si 2p state successively shifted to higher energy while the binding energy of the N 1s state showed no change with the increase of the N-2/Ar ratio. The films deposited under conditions of N-2/Ar ratio below 0.5 were silicon-rich while the films formed with the N-2/Ar ratio more than 0.5 were nitrogen-rich. Nearly stoichiometric Si3N4 films can be obtained at the N-2/Ar ratio of 0.5. Grazing incidence X-ray reflectivity showed that the nearly stoichiometric Si3N4 film had a higher density (3.17 g/cm(3)) while the silicon-rich or nitrogen-rich resulted in a decrease in film density. The surface roughness of these films lies almost in the same level.