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A simple and new relative humidity (RH) capacitive sensor with a two-layer planar structure based on anodic aluminium oxide (AAO) film serving as a sensing layer has been fabricated. A pair of separated tantalum thin films under the AAO layer served as two coplanar electrodes. AAO film was fabricated by anodising deposited aluminium thin film in oxalic acid. The influence of the pore size and thickness of the AAO film, and the electrode spacing, on the performance of the sensor was investigated. The RH sensor with a thicker AAO film and narrower electrode spacing was more sensitive. By pore widening, the sensitivity of the sensor can be improved. The achieved average sensitivity as high as ~3.57 pF/RH% was obtained for sensors with an AAO layer of 2 μm thickness and 80 nm pore size. These results were explained using the partial capacitance method and parallel-serial model. In addition, under the humidity range of 20-80%, sensors with 70 nm pores had the highest sensitivity, which was interpreted in terms of effective permittivity and anion concentration. This study provides an easy method to fabricate high-performance humidity sensors with very simple structures for practical applications.