First-principles calculations based on the density functional theory are used to study the crystal structure, electronic and optical properties of Nb doped anatase and rutile TiO2. The calculated results reveal that anatase TiO2:Nb has a smaller effective mass and carriers nearly twice lager than those of rutile TiO2:Nb under the same doping concentration. And anatase TiO2:Nb also exhibits a greater room-temperature ionization of donors. Besides, the calculated optical properties indicate that anatase TiO2:Nb has a more excellent transparency than rutile TiO2:Nb. All the results suggest that anatase TiO2:Nb is more applicable to transparent conductive oxides. The calculated results consist well with the available experimental results.