We report that carbon nitride thin films can be formed by microwave plasma-enhanced chemical vapor deposition (PECVD). Gas mixtures containing CH4, H2, and NH3 in various ratios were tried as the precursors, and a Si(100) wafer was used as the substrate. The films were characterized by X-ray photoelectron spectroscopy (XPS), and electron microscopy (both SEM and TEM). A Si content of about half of the carbon content was observed when the substrate temperature exceeded 1000 °C. Microscopic investigation revealed the coexistence of large-grain (over 10 μm) and fine-grain (under 1 μm) crystals. We suggest the presence of a crystalline carbon nitride phase corresponding to an α-C3N4 structure (isomorphic to α-Si3N4), which may also be a stable hard material.