In this study, we investigate the dependence of superconducting transition temperature (T c) on sheet resistance (R sq) for two series of NbN films with different homogeneities which were prepared on either MgO substrates (NbN/MgO) or MgO substrate covered with a 1nm thick AlN layer (NbN/AlN/MgO). The R sq-dependent T c (T c (R sq)) behavior of the homogeneous NbN/MgO films could be explained with Finkel'stein's formula, which is based on localization theory using reasonable fitting parameters. On the other hand, the T c (R sq) values of the NbN/AlN/MgO films were larger than that of the NbN/MgO films in the region To understand the origin of this deviation from the theory for homogeneous systems, we assumed that the NbN/AlN/MgO films have the potential to form an inhomogeneous percolation structure. To confirm this assumption, the temperature (T) dependence of the upper critical magnetic field ( near T c was analyzed using the form which is based on classical percolation theory. From the dependence of R sq ((R sq)) of both series, it was found that the (R sq) of the NbN/AlN/MgO films deviates in the region from in a homogeneous NbN/MgO films. By introducing an effective sheet resistance to account for the inhomogeneous structure of the NbN/AlN/MgO films, we were able to qualitatively explain the differences between the T c (R sq) behaviors of the two series.