We have found experimentally that the stress in a-Si:H films is Compressive, and tensile in a-SiNx:H film on the flate silicon substrate with certain content of Si-H bond. The stress in a-Si:H/a-SiNx:H/c-Si system can be made to reach a minimum by suitably chosing some conditions of depositing. The minimum stress may keep for a long time. It is shown that the stress changes with annealing temperature. Some explanation to these results are also given.