The influence of high temperature annealing on change of physical and chemical properties, especially crystalline structure of silicon carbonitride layers were investigated by optical techniques (ellipsometry, IR and Raman spectroscopy), SEM and diffraction of synchrotron radiation. SiCN films were synthesised by remote plasma enhanced chemical vapour deposition at total pressure of 6x10(-2) Torr and at temperatures of 473-1173 K using mixture of the helium and hexamethyldisilazane as a volatile single-source precursor. It was established that as-grown films consist of amorphous and nanocrystalline (20-90 Angstrom) components of the pseudo alpha- Si3N4 phase. Thermal annealing leads to ordering of structure, increase of density and crystal size exceeding 1000 Angstrom.