Hydrogen-free SiNx films were deposited at N2 flow rate ranging from 1 sccm to 20 sccm by microwave electron cyclotron resonance plasma enhanced unbalance magnetron sputtering system. We studied the influence of N2 flow rate on the structural characteristics of deposited films in chemical structure, stoichiometry, composition at different depths in film, and hardness by using X-ray photoelectron spectroscopy and nano-indantation. The results indicate that the films deposited at low N2 flow rate are Si-rich structure. The films deposited at 2 sccm N2 flow rate show an excellent stoichiometry with 94.8% Si—N bond content and uniformity of composition in different depths. At the same time, the films display the highest hardness value of 22.9 GPa. The films deposited at high N2 flow rate contain too much N—Si—O bond and Si—O bond, which is caused by chemical absorption both on and in film in atmosphere. The films present N-rich structure. In this situation, the films display poor mechanical properties with hardness of only 12 GPa.