The origin of the negative temperature coefficient of the resistivity (TCR) for ultra-thin YBa2Cu3O7-δ films (d≤5 nm) is closely related to their granular structure. If the mean free path of the system is larger than the averaged grain size, then the metallic behavior of the Boltzmann conductivity will be modified by the effect of grain boundary scattering in which an effective mean free path Lg can be defined. This model predicts that the TCR changes from positive to negative at some critical temperature and the resistivity is saturated in the low temperature region. An anomalous broadening of the superconducting transition is supposed to be resulting from the inhomogeneity of the composition, e.g., due to oxygen loss, which in conjunction with the grain boundary scattering can successfully elucidate the exotic TCR of the ultra-thin superconducting films. A similar phenomenon is also observed in radiation-damaged films with thicknesses of 0.5 μm, implying that this anomaly does not appear as an intrinsic characteristics of the material caused by the film thickness.