Experiments are carried out on the frost and defrost characteristics of fine tube bundles under coolant temperature between -20 and -58 degrees C. The diameter of the tube is 2 mm. The frost and defrost processes are observed by taking photos and measuring pressure drops. The results indicate that the frost layer thickness on the tube surface increases with decreasing coolant temperature, but decreases with increasing air humidity. A model is given to explain the variation of frost thickness. The experiments verified that spraying ethanol is effective on inhibiting the formation of the frost. (C) 2017 Elsevier Ltd. All rights reserved.