This study investigated the effect of low-temperature heating on internal stresses of SiNx thin films prepared by plasma enhanced chemical vapour deposition. It was found that films deposited at 473 K developed tensile stresses after heating in air to temperatures above 573 K. The stresses increased with increasing heating time and temperature. Heating to above 873 K was found to cause partial apparent relaxation of the tensile stresses and visible physical damages, in the form of circular cracks. Young's modulus and hardness of the annealed films were determined by means of nanoindentation method. It was found that both Young's modulus and hardness of the films increased after heating, and the increments also increased with increasing heating temperature. These observations imply that changes in chemical structure of the films have been induced by heating in air. These changes are attributed to out gassing of the films caused by the dissociation of hydrogen bonds.