A method to calculate the optical band-gap of thin films composed of both crystalline phase and amorphous phase is suggested. Chromium oxide films were taken for the optical band-gap measurement, and the results were compared with those obtained by the conventional method. There is discrepancy between the values calculated by two different methods. The discrepancy decreases as the deposition temperature increases to the point where crystalline peaks occur. It has been speculated that the simple Tauc's relation for pure amorphous film would lead to an underestimation of the fundamental band-gap of the film composed of both amorphous and crystalline phases. A modified Tauc's equation, δ=f1δc+(1−f1)δa, approach suggested by Krankenhagen et al. [J. Non-Cryst. Solids 198–200 (1996) 923] yields optical band-gap of the Cr2O3 film in the range of 4.7~5 eV when the deposition temperature was varied from room temperature to 300°C.