Temperature-dependent reflection measurements have been carried out on InN thin films to determine its refractive index n and its temperature coefficient dn/dT. The studied InN thin films were grown by reactive sputtering on (111) GaAs substrates under various growth conditions. In addition to the slow variation of the background in the region far away from the optical band gap, a rapid increase of dn/dT due to the Wannier excitonic effect has been observed in the region near the optical band gap. With the help of the dielectric function model for the refractive index n and Tanguy’s model for the temperature coefficient dn/dT [ J. Appl. Phys. 80, 4626 (1996) ], we are able to demonstrate the dependence of dn/dT in InN thin films on the growth temperature and on the sputtering pressure. The results are discussed within the framework of the optical-band-gap fluctuation and the reactive-sputtering growth mechanisms.