Abstract: Thin films of cadmium sulfide with different thickness (300-600 nm) were deposited on preheated glass substrates (430°C) by Ultrasonic Nebulizer Deposition (UND). The effect of thickness on the structural, optical and gas sensing properties of films has been investigated. X-ray diffraction patterns indicated that the CdS films had a polycrystalline hexagonal structure with preferred orientations along (002) and (101) directions. The crystallite size increases along (002) plane with the thickness increase and attains a maximum 56.68 nm for 600 nm. The surface roughness is decreased with the increase of the films thickness which has been investigated by Atomic Force Microscopy (AFM). It was observed that as the film thickness increases from 300-600 nm the optical band gap blue shifted from 2.35-2.42 eV. The sensing characteristics of the films were studied as a function of operating temperature and for three different concentration (10-30 ppm) of ethanol. The results showed that the sensitivity increases with operating temperature and reached a maximum values 75% at 310°C for the film with 500 nm thicknesses. Moreover, the films showed fast response and recovery times at higher operating temperature.