The dielectric properties of (A3+, Nb5+) co-doped TiO2 (A = Al, Ga and In) ceramics prepared by a solid state reaction method were studied. Sintered (A3+, Nb5+) co-doped TiO2 ceramics with A = Al and Ga were found with a dense ceramic microstructure. Besides the internal barrier layer and surface barrier layer capacitor effects, the formation of electron pinned defect dipoles at a specific region may have probably an influence on the dielectric response in these co-doped TiO2 ceramics doping with different A3+ dopants (A = Ga and In). All of the sintered ceramics exhibited giant dielectric permittivity ranging from 103–104 and low loss tangent (tanδ≈0.05 at 1kHz). To understand the cause of large dielectric permittivity in these materials, the theoretical modellings based on density functional theory were carried out. Our calculation results revealed that triangular shape of A2VOTi (VO=oxygen vacancy) was found in (A3+, Nb5+) co–doped TiO2 with A = Ga and In. Moreover, the diamond shape of 2Nb doped TiO2 was observed to be the most preferable configuration. These shapes may generate large defect-dipole clusters in this structure and hence, the dielectric constants are dramatically improved.