Hydrogenated silicon nitride deposited by LF-PECVD is commonly used for anti-reflection coating and passivation in silicon solar cell fabrication. The deposition of the optimized silicon nitride on the surface is elemental in crystalline silicon solar cell. In this work, the carrier lifetimes were measured while the thicknesses of SiNx were changed from 700Å to 1150Å with the gasflow of SiH<SUB>4</SUB> as 40 sccm andNH<SUB>3</SUB> as 120 sccm,. The carrier lifetime enhanced as the thickness of SiNx increased due to improve dpassivation effect. To study the characteristics of SiNx with various gas ratios, the gas flow of NH<SUB>3</SUB> was changed from 40 sccm to 200 sccm with intervals of 40 sccm. The thickness of SiNx was fixed as1000Å and the gasflow ofSiH<SUB>4</SUB> as 40 sccm.The refractive index of SiNx and the carrier lifetime were measured before and after heat treating at 650°C to investigate their change by thefiring process in solar cell fabrication. The index of refraction of SiNx decreased as the gas ratios increased and the longest carrier lifetime was measured with the gas ratio NH<SUB>3</SUB>/SiH<SUB>4</SUB>of 3.