The underlying mechanism of heat transfer for several sub-processes that control a bubble growth cycle during flow boiling in microchannels is studied in this article by means of high frequency measurements of liquid film thickness and temperature accompanying synchronous visualization. The test section is made up of glass tube having an internal diameter of 0.94 mm with the Indium Tin Oxide (ITO) electrically conductive layer as heaters. The initial liquid film thickness formed by a boiling bubble slug agreed well with Taylor's law. For most cases of the liquid film thickness evolution, the thinning of the liquid film was found to be much quicker than the prediction when considering evaporation effect alone. A new model for predicting the liquid film thickness evolution under flow boiling condition was developed by inclusion of the combination effect of evaporation and shear stress. It was found that the sub-processes of a bubble growth cycle were dependent on heat flux. The cyclical fluctuation of the temperature was due to the different heat transfer capability of the sub-processes. The time ratio and relative importance of the different heat transfer mechanisms were quantitatively estimated by linking the transient temperature fluctuations with the liquid film thickness variations. (C) 2017 Elsevier Ltd. All rights reserved.