Amorphous quaternary alloy (a-SiCN:H) films composed of silicon, carbon, nitrogen and hydrogen have been prepared by the rf glow-discharge decomposition in a gas mixture of methane, silane, nitrogen and helium. Effects of nitrogen addition on the optical, electrical, structural and optoelectronic properties of the films have been investigated. Optical bandgap remains almost constant for a wide range of nitrogen addition. Dark and photoconductivity becomes greater by the nitrogen addition than those of undoped a-SiC:H films. Incorporation of a small amount of nitrogen in the film is likely to reduce the structural disorder and/or the density of defects, and some nitrogen atoms seem to work as a dopant. On the other hand, large incorporation deteriorates these properties. The nitrogen addition effect appears more remarkably in the films prepared under the low flow rate ratio of methane to silane.