MnBi_0.95Sb_0.05 alloys were prepared by the conventional melt technique and then deposited as a thin film by thermal evaporation technique with different thicknesses. The optical absorption, reflection, and transmission spectra of thin films were measured in the wavelength range of 250 nm to 2500 nm. In our work, MnBi_0.95Sb_0.05 thin films have a hexagonal structure (P63/mmc group). The optical properties are characterized by high absorption and reflection, as well as very low transmission. Besides that, the absorbance of MnBi_0.95Sb_0.05 films decreased with increasing thickness. The calculated energy gap of MnBi_0.95Sb_0.05 thin films depended on the film thickness. It decreases from 1.36V to 1.03V by increasing the thickness from 25nm to 75nm. Different optical parameters like n, k, and ϭ_opt are examined and discussed.