Heat transfer measurements were performed in a mini/micro-channel boiling system for water, and the thickness of the micro-layer that formed between the heating surface and the generated vapor under vapor growth was measured by application of the laser extinction method for narrow gaps of 0.5, 0.3 and 0.15 mm. The process of bubble growth was recorded by using a high-speed camera simultaneously. The effects of gap size, the velocity of vapor bubble forefront and the distance from the incipient bubble site were investigated on the micro-layer thickness in a narrow gap mini/micro-channel boiling system and the configuration of the thin liquid micro-layer distributions on the heat transfer surface was clarified. Furthermore, factors that would possibly affect the mechanism and characteristics of heat transfer, such as the position of the generated vapor bubble, the velocity of the vapor forefront, the micro-layer dominant period and the liquid saturation period in the boiling cycle, and so forth, were quantitatively investigated and analyzed on the basis of the measured data. The heat transfer characteristics were analyzed and the data calculated were coincided with the measured data in the boiling curve for the gap size of 0.5 and 0.25 mm measured in the previous report. It was shown that the heat transfer was enhanced due to the micro-layer evaporation. (C) 2008 Elsevier Ltd. All rights reserved.