Bubble behaviors are closely related to the heat transfer performance during flow boiling in microchannel, however, the effect of channel cross-section decreasing on the bubble growth is still not fully understood at present. In this work, an experimental investigation is conducted to investigate the bubble growth characteristics during flow boiling in a single microchannel with 0.5 mm x 1 mm rectangular cross-section, and the heat transfer performance of flow boiling and its influencing factors are studied. Experiments are conducted with subcooled deionized water and the bubble behaviors are visualized by a high speed CCD camera installed upon the test section. Depending on the heat flux, different growth features are observed in the bubble growth process. Two kinds of bubble growth model are identified: the power law model in initial growth period and the linear law model in later period. The confinement effect of the microchannel is deemed as the mechanism causing the alteration of bubble growth models during its growth process. The deformation features of confined bubble are discussed to illustrate the intensification of evaporation on the liquid-vapor (LV) interface at bubble root, which increases the growth rate of bubble in its confined growth period as well as the heat transfer capability of bubble. Therefore, the maximum local heat transfer coefficient along the channel is found in the region where confined bubble and/or short elongated bubble flow pattern are dominant. Moreover, the heat flux is found to have great influence on the overall heat transfer performance of flow boiling in microchannel, but the effect of mass flux is much less. (C) 2016 Elsevier Ltd. All rights reserved.