Abstract: The nanocrystalline CdS was preparaed by coordination-precipitation method in the system of citrate. The results from X-ray diffraction（XRD） indicate that the products are of cubic zinc blende structure,the mean size of the CdS powders is about 25 nm. The gas-sensing characteristics were studied and the results reveal that the sensors based on CdS show a high sensitivity and selectivity to ethanol gas. And its sensitivity to 0.005 vol% ethanol gas can reach 45 at 230℃.