The effects of nitrogen content on the interfacial properties in a-Si:H/a-SiNx:H superlat-tices have been studies by electroabsorption (EA) photothermal deflection spectroscopy (PDS), photoluminescence (PL) and infrared (IR). The density of interface charges Qs deduced from EA is approximately 1012cm-2. The density of interface states Ni estimated by PDS is ～1011cm-2 which is smaller by a factor of 5 than Qs. The nitrogen content x dependences of Qs, Ni and the relative peak intensity of luminescence Ipl exhibit similar behavior and show extreme values at x=xc which is defined as critical value of nitrigen content. The critical values of nitrogen content are in the x range of 0.85-0.95. The measurements of IR reveal that the absorption of N-H bonds increases rapidly with increasing x in the range of 0.6-1.0. The asymmetry of interfaces and a possible mechanism of presence of critical values are discussed-For interpretating the phenomena observed, a model of charged nitrogen dangling bond is suggested.