In this work, the study of annealing effects on electrical properties of Zinc Oxide Thin-film Transistors is presented. The samples were annealed at 180°C under Nitrogen ambient. The conductivity and contact resistance of ZnO films with Aluminum electrodes are studied by Transmission Line Method. Also, the Zinc Oxide films obtained by ultrasonic spray pyrolysis at 200°C are studied by X-ray diffraction and Fourier transform infrared spectroscopy. A comparison of the electrical properties as a function of annealing time is presented. The results show an optimal annealing time and after this time, the metal-ZnO interface deteriorates. •The study of annealing effects on aluminum/ZnO contacts is presented.•There is an optimal time of annealing and after this time, the contact tends to degrade.•ZnO bonds clearly shows a reduction as the annealing time is increased.•Output characteristics present a clear saturation regime after 30min of annealing.