The optical band gap is a basic property of optical materials. The measured band gap depends not only on the material but also on its characteristics such as cryslallinity and stoichiometry. The optical band gap of chromium aluminum oxide films was measured and calculated by three different methods. Firstly, we used the conventional experimental-graphical method, which is commonly used. However, this method is applicable only to an all-crystalline phase or an all-amorphous phase. The second one was an experimental-calculation method applicable to films composed of both crystalline and amorphous phases. We calculated the optical band gap between the highest occupied molecular orbital of O2p and the lowest unoccupied molecular orbital of Cr3d in Cr1.71Al0.29O3 films composed of both amorphous and crystalline phases. Also, a band gap fur the d-d transition was obtained. Finally, the measured value was compared with the theoretical optical band gap calculated by the discrete variational-X alpha (DV-X alpha) method.