The electrical characteristics of rutile- and anatase-type TiO2 films were investigated by supplying alternatively pulses of trimethylaluminum (TMA) and H2O vapours after AI2O3 deposition using atomic layer deposition (ALD). As-grown rutile- and anatase-TiO2 films had an insulative property of ∼105ωcm, but its resistivity dropped dramatically to 10-1-10-2 ωcm and stayed low after supplying 5 cycles for ALD of AI2O3. The carrier density values of both TiO2 films were 1019-1020 cm-3 in the low-resistivity region. The Hall mobility values were 0.5-15 cm2V-1s-1, which correspond well with the literature values of 0.2-6 and 10 cm2V-1s-1 for thin-film and single-crystal anatase-TiO2, respectively. We propose one mechanism that oxygen vacancies must be formed by the reaction of adsorbed TMA precursor and the oxygen in TiO2 film after ALD-AI2O3 deposition and leads to electron generation, and results in change of resistivity from an insulative into a semiconductor characteristic.