Al doped ZnO films have been prepared by dc magnetron sputtering. These films were annealed in different atmosphere and temperature. The crystal structures were analyzed by x-ray diffraction (XRD), and the magnetic properties were measured by a Physical Properties Measurement System (PPMS) with the magnetic field paralleled to the films plane. The results show the microstructure and magnetic properties were influenced by annealing atmosphere. Compared to the films annealed in vacuum, the films annealed in air shows obvious room temperature ferromagnetism, the magnetic moment increases about an order of magnitude. The room temperature ferromagnetism may be associated with a charge transfer between Al and Zn and the variational position of Al in ZnO films in different annealing ambience.