We introduce a methodology to estimate the nonlinear capacitance of interfacial "passive" layers of ferroelectric thin films from domain switching currents directly. The methodology has the advantage over the traditional extrapolation technique of a linear plot of the inversed capacitance against the film thickness which neglects the size effect on the ferroelectricity. Expectedly, this technique remains suitable in ultrathin films with the thickness scaling down into a few nanometers, where the size effect is important. From our measurements, we found that the interfacial capacitance increases nonlinearly with the reduction of the applied voltage in a tendency similar to the capacitance-voltage curve of ferroelectric thin films above the coercive voltage. Nevertheless, the capacitance at a high field drops down closely to a value derived from the traditional extrapolation technique. The pertinent physics is discussed in this work. Finally, we observed the reduction of the interfacial capacitance with the rising temperature, which suggests the thickening of interfacial layers at high temperatures. (C) 2008 Elsevier B.V. All rights reserved.