The SiCN film, which has the same crystal structure as 3C-SiC verified by X-ray diffraction and Raman scattering, has been formed on Si (111) substrate by constant-source diffusion using a chemical vapor deposition system. Via X-ray photoelectron spectroscopy, the film has been identified as the mixture of elemental Si and SiC (1)_(X)N(X)alloy, with the SiC (1)_(X)N(X)content decreases from 87.4% to 17.9% and the mole fraction x increases from 0.17 to 0.21, as the depth extends from the near surface to the interface. The measured composition distribution can be well explained by the constant-source diffusion model, confirms that the formation mechanism of the SiCN film is the diffusion of C and N into the Si substrate. This provides a new method for fabricating crystalline SiCN thin films.