A rapid response relative humidity sensor based on nano-structured aluminum oxide thin film is disclosed. The main body of the sensor is an anodic aluminum oxide thin film of about 2-20 μm thick formed from an Al substrate which serves as one electrode. A porous metal layer of about 20-200 nm thick is formed over the anodic aluminum oxide thin film and functions as a second electrode. The alumina thin film contains nano-sized channels of about 10-100 nm in diameter, serving as a capacitive relative humidity sensor having high sensitivity and short response time. Thermal annealing at elevated temperature improves the linearity of the capacitance versus humidity curve, giving a full range humidity sensing range. Hysteresis and degradation are negligible for the humidity sensors.