The present study investigates the microstructure and bactericidal ability of polycrystalline and epitaxial TiO2 thin films with anatase and rutile structure. Phase formation and the distortion of the lattice of the film compared with bulk are proved by TEM observation. HRTEM images assure the good crystal quality and compact structure of the films prepared by rf helicon magnetron sputtering. This obtained larger band gap due to the lattice deformation could contribute to the better photocatalytic effect of the anatase film. Bactericidal abilities evaluated by the photokilling E. Coli using film stick method under UV irradiation show the following result: Both single and polycrystal anatase films exhibit obvious bactericidal abilities while no distinguishing difference for them. On the other hand, no bactericidal ability is observed for rutile films in this work. The band gap value of rutile film is lower than the energy potential required to O-2/O-2(-) reductions according to the principle proposed by Fujishima et al. This resulted in the observed poor photocatalyst effect of rutile film. (C) 2004 Elsevier B.V. All rights reserved.