The thermal expansion coefficient of a-CNx:H thin films was determined by the thermally induced bending technique. The films were deposited by glow discharge under methane and nitrogen atmosphere, and analyzed by FTIR and Raman spectroscopies, nanohardness, and stress measurements. Drastic changes of the film structure were observed as a result of the nitrogen incorporation, from 0 to 7%. The increase of nitrogen concentration reduces the deposition rate, stress, hardness, and the elastic constant of the films. It was also observed that the thermal expansion coefficient has a significant increase from approximately 2-9 x 10(-6)/K, which was associated with the increase in the sp(2) concentration induced by the N incorporation, and with the increase in the concentration of C-N bonds. In spite of that, stable and thick ( similar to 2 mum) films were deposited at moderate deposition rate (0.3 nm/s), relatively high hardness (13 GPa), and low stress (0.6 GPa).