Visualization study using a high speed camera was carried out to explore the mechanisms of net vapor generation (NVG) in subcooled flow boiling. The flow direction was vertical upward, the working fluid was water, and the experiments were conducted under moderate pressure conditions; a rather hydrophilic surface was used as a heated surface. In all the experimental conditions tested in this work, the bubbles nucleated on the heated surface typically departed from the nucleation sites and then slid upward on the vertical heated surface. When the NVG condition was exceeded, the number of bubbles increased noticeably and the range of bubble size became wider with an increase in the thermal-equilibrium quality. It was however found that the rapid increase of the void fraction after the NVG condition was reached could mainly be attributed to the formation of a small number of large bubbles. It was also observed that the large bubbles were mostly produced at the nucleation sites immediately after other sliding bubbles passed over the sites. It was therefore supposed that the production of large bubbles in the wake region that was formed behind the preceding sliding bubbles was a key phenomenon in causing the NVG in the present experiments. (C) 2013 Elsevier Ltd. All rights reserved.