Silicon nitride thin films were deposited on K9 glass by r.f. magnetron sputtering without heating. Comparisons of the optical properties were made as the films′ deposition conditions changed. Several different deposition routines showed that gas ratio played much more important role than total gas pressure when total pressure is low. The turning point of Si-rich film and N-rich film lies at the flow rate N 2-to-Ar of 3∶14 . Keeping total pressure low is necessary for the sake of deposition rate and refractive index. SiN x thin films were studied by AFM and UV-VIS scanning spectrophotometer.