Ultrathin anatase TiO2(001) films have recently been shown to exhibit many exotic properties, which are not observed in their thick counterpart. In this work, the dependence of the geometric and electronic properties of ultrathin anatase TiO2(001) films on the number of O-Ti-O trilayers is investigated on the basis of first-principles calculations. It is interesting to find that the lattice parameters, intertrilayer distances, electronic band gap and the position of the valence band edge for the films depend strongly on the number of trilayers, and they exhibit pronounced odd-even oscillations with the number of trilayers. Moreover, the convergence of geometric and electronic properties with the number of trilayers is rather slow, and not achieved even in films with 12 trilayers (∼2.7 nm). In addition, the adsorption state of the H2O molecule on the surface depends strongly on the thickness of the film. On the other hand, because the TiO2(001) surface is well described by a (001) film containing more than four trilayers, films with different lattice parameters and more than four trilayers may also be regarded as (001) surfaces that have been strained to different extents. Then, the above results from them also apply to the strained TiO2(001) surfaces. These results not only present new physics of the TiO2(001) films and surfaces, but also are helpful for understanding and modulating their performance in photocatalytic water splitting.