The influences of the substrate temperature and post deposition annealing in an Ar gas flow on the transmission, reflection and absorption spectra, optical constants and carrier concentration of CdIn 2O 4 thin films, which was deposited by r.f reactive sputtering from a Cd In alloy target, are discussed. It shows that carrier concentration decreases when substrate temperature is increased, and concentration will increase after thin film is subjected to annealing. The peaks of refractive index n and extinction coefficient k undergo blue shift with increasing substrate temperature, and red shift occurs after thin film is annealed. This is in good agreement with the change of carrier concentration. It is of significance to monitoring in situ the film process in the course of deposition of the transparent conducting thin films such as CdIn 2O 4 which are sensitive to deposition methods and deposition conditions.