The Urbach bandtail characteristics in InN thin films grown by radio-frequency magnetron sputtering on sapphire (0001) substrates have been investigated both theoretically and experimentally. The bandtail parameter in InN thin films has been obtained by temperature-dependent transmission spectra, with the aid of a detailed calculation of the transmission profile. A bandtail model based on the calculation of density of occupied states and the carrier-phonon interaction has been employed to analyze the temperature-dependent bandtail characteristics. The bandtail parameter is in the range of 90-120meV in the InN thin film. It is found that the carrier-phonon interaction in InN is weak and the structural disorder contribution (～90meV) dominates over the interactive terms. The high structural disorder in InN thin films may relate to the high nonradiative recombination centers.