In a - Si : H / a - SiN x: H multilayers, in addition to induced distortions, there are a large number of induced defects in the interface regions between a-Si:H and a - SiN x: H due to structural mismatch. In this experiment, we measured a series of a - Si : H / a - SiN x: H (x= 0.5) multilayers by the positron annihilation technique (PAT) and, on the basis of the measured results, present a structural model of the sublayers in the multilayer. Using this model, we have obtained theoretically the relative positron annihilation intensity I2 values of multilayers. The calculated values are in good agreement with the measured values. By this experiment, it is confirmed that the interface region structures on both sides of the sublayers are asymmetric and related to the growing direction of the film. Moreover, the following information in the a-Si:H sublayer is gained: There are a larger number of induced defects in the interface region away from the substrate, but few or no in the interface region near the substrate; The induced defects in the interface region are situated in the subregion not close to the interface but at a little distance, about 8 Å, from the interface, the thickness of the subregion is about 50 Å. The induced defect density in the subregion is estimated to be about 1011/ cm 3.