Titanium dioxide (TiO2) thin films are deposited on two different roughness K9 glass and monocrystalline silicon substrates by electron beam evaporation technique and all samples are treated with oxygen ion beam. Surface roughness of substrates and all samples are measured using Talysurf CCI. The refractive index of TiO2 thin films is measured by an ellipsometer. The experimental results show that TiO2 thin films decrease the subatrate surface roughness,and ion beam post-treatment improves the surface roughness of TiO2 thin films. When the substrate roughness is larger,with the increase of ion energy,the surface roughness of TiO2 thin films decreases firstly,and then increases;with the increase of ion beam bombardment time,the surface roughness of TiO2 thin films decreases obviously. With the increase of ion beam density,the surface roughness of TiO2 thin films decreases. The surface roughness of all samples is also decreased with ion beam treatment.