We have studied the characteristics of amorphous carbon nitride (a-CNx) films as low dielectric constant materials for ultralarge scale integration. In this article the initial internal stress of a-CNx films is studied and discussed. The stress in a single-layer film on a substrate is determined using the measured radius of curvature and Stoney's equation. a-CNx films are prepared by a reactive radio frequency magnetron sputtering on the ultrathin quartz glass substrates. It is observed that the internal stress in a-CNx depends mainly on the substrate temperature increasing from about 10 to 50 MPa of compressive nature. These values are about one tenth of that for hydrogenated amorphous silicon (a-Si:H) prepared by a plasma-enhanced chemical vapor deposition.