Highly sensitive thick film ethanol gas sensors based on a nanocrystalline $In_{2}O_{3}$ were fabricated by painting method on alumina substrates. The crystal structure of the $In_{2}O_{3}$ powder was characterised by XRD analysis. The microstructure of the films were characterised using FE-SEM. The experimental results of the ethanol gas sensing characteristics indicated that the undoped $In_{2}O_{3}$ thick film has a high sensitivity. The sensitivity of the film heat treated at $400^{circ}C$ for 2 hrs. was as high as 32.73 at an operating temperature of $200^{circ}C$ to 1000 ppm ethanol gas in air.