Thermal effects on the crystal structure, electrical and optical characteristics of the Al and F co-doped ZnO films (ZnO:AlF3) are discussed in the paper. The ZnO:AlF3 thin films are prepared by RF sputtering with a constant power (ZnO/AlF3=100W/75W) toward the ZnO and AlF3 targets. The substrate temperature varied from room temperature to 250 °C with a step of 50 °C during thin film deposition. The crystalline quality of the ZnO:AlF3 film improved as the substrate temperature increased, with a corresponding increase in grain size. The improvement of the film quality leads to a higher electron mobility, with electron mobility of 0.85 cm2/V-s for the film deposited at the substrate temperature of 250 °C. The doping effect of fluorine in ZnO, and hence carrier concentration, was reduced at high temperature due to the vaporization of fluorine. This led to a reduction of carrier concentration with increase of temperature from 25 to 200°C. The corresponding resistivity increased from 3.60×10−2 to 6.0×10−2 Ω-cm. While for a further increase in substrate temperature, the doping of Al to the ZnO film was increased and resulted in an increase in carrier concentration.