Co-doped zinc oxide (ZnO) films were fabricated by oxidizing Zn/Co bilayer thin films prepared by vacuum evaporation. The effects of oxidation temperature and high magnetic field on the structure and optical transmittance were investigated. The results show that Co-doped ZnO films in the absence of a magnetic field prefer to grow in a coral dendrite-like morphology. This morphology is inhibited by the Lorentz force of a high magnetic field. Elevating the oxidation temperature to 600 degrees C from 450 degrees C can lead to more Zn ions being substituted by Co ions. However, the increase in Co substitution can be inhibited by a high magnetic field. According to X-ray photoemission spectroscopy results, the chemical states of Co are affected by the oxidation temperature and a high magnetic field. Co2+ distributes uniformly in the host ZnO lattice at 450 degrees C. However, CoO and Co3O4 form at 600 degrees C without a magnetic field. The obvious energy differences for Co 2p indicate that the Co chemical state changes with the application of a high magnetic field. These results thus show that a high magnetic field can be used to modify the state of Co in ZnO films. Optical transmittance data show that the Co state, O vacancies, and crystallinity have no obvious influence on the transmittance, which is strongly related to the dendrite-like structure. (C) 2015 Elsevier B.V. All rights reserved.