Hydrogen-free SiNx films were deposited at substrate temperature ranging from room temperature to 700℃ by microwave electron cyclotron resonance plasma enhanced unbalanced magnetron sputtering system. We have studied the influence of substrate temperature on the structural characteristics of deposited films including growth rate, microstructure, grain size, and hardness by using transmission electron microscopy, Fourier-transform infrared spectroscopy, and nano-indentation. The results indicate that the films deposited at room temperature are amorphous, and α-Si3N4 grains with random epitaxial sizes appear when substrate temperature is higher than 300℃. The α-Si3N4 grain size increases with substrate temperature up to 620℃, and then decreases at 700℃. At 700℃, the grains have uniform epitaxial sizes, and value of the film hardness reaches the maximum (36.7GPa).