A series of a-Si:H/a-SiNx:H(x=0.5) multilayers are studied by positron annihilation technique (PAT). It is found that a large number of defects are induced in the interface region of a-Si:H/a-SiNx:H multilayers, owing to the structure mismatch of a-Si:H and a-SiNx:H. In the a-Si:H sublayer, there is a strained layer close to the interface, its thickness is about 8?, and a transition layer above the strained layer, its thickness is about 50?. There are a large number of defects in the transition layer, they are called the "interface defects".