The present project work has mainly focused on optical properties of undoped and tin doped indium oxide (In2O3) Nan-particleS. In this project, the band gap, crystallite size and index of refraction of ITO nano-particle were calculated from UV-Vis spectra using Brus model, Deby Scherer‟s and mos‟s relation. As the tin concentration increased from 5% to 50%, the calculated energy band gap decreased from 3.726 eV to 3.709 eV and the crystallite size is increased from 18.40 nm to 25.68 nm. The band gap energy of undoped and Sn doped indium oxide (In2O3) nano-particle is inversely proportional to the index of refraction calculated by Moss relation formula. So, the calculated refractive index of ITO is increased due to the decrease of band gap energy as the tin concentration increases.