The Mo-Al2O3 cermet granula films were deposited on stainless steel substrates by magnetron sputtering. The Maxwell-Garnett equivalent medium theory was used to analysis the optical constants of Mo-Al2O3 granular films. The optical constants, such as n and k, and their variations with wavelengths of from 300 nm to 800 nm were studied for the Mo-Al2O3 granular films based on the optical constants of bulk material and thin films of Mo and Al2O3. The possible reasons for the disagreement between theory and experiment were explained at the same time.