We report here on the synthesis of large crystals of Si-containing carbon nitride, consisting of a predominantly C-N network, by microwave CVD. The Si content in this material varies from crystal to crystal and also with the deposition conditions and has been observed to be as low as less than 5 at. % in some crystals, wherein the Si atoms are believed to substitute for some of the C sites only. This is the first time that such large and well-faceted crystals consisting almost entirely of carbon-nitride network have been synthesized. Moreover, there is no obvious deposition of amorphous CN material.