A series of Al-doped ZnO (AZO) structures, including disk-like, flake-like, flower-like and dumbbell-like morphologies, have been synthesized by a hydrothermal method without any catalyst or template. The morphologies of these AZO structures can be conveniently controlled, by selecting the additives and controlling the experimental conditions, and the result has excellent reproducibility. Gas sensors based on AZO were fabricated and the gas sensing properties were investigated. The sensors showed high response values and reproducible response–recovery for 5–500 ppm ethanol at 332 °C, compared with NH₃, CH₃OH, H₂, CO, HCHO, NO_x and a specific mechanism was proposed.