In this study, bubble dynamic and the mechanisms to cause net vapor generation (NVG) were explored experimentally in a rectangular vertical upward subcooled flow boiling under atmospheric pressure, and new results was found on various conditions of surface wettability. In the course of observation, two different vapor bubble behavior were observed and in low void fraction region new mechanism for incipience of net vapor generation was proposed. On a hydrophilic heated surface, at boiling incipience all the bubbles were lifted off the heated surface at atmospheric pressure and immediately collapsed in the subcooled liquid. On the contrary, when the surface was hydrophobic, bubbles mostly stuck on the nucleation sites at ONB condition. Furthermore, in this study, experiments were performed using rather hydrophilic and hydrophobic heated surface to propose the new mechanisms of NVG. An important result revealed in this work was that on a hydrophobic heated surface with high contact angle around 90°, bubble departure from all the nucleation sites which is a necessary condition to cause NVG, occurs in proximity to onset of significant void (OSV). The direct cause of OSV for the hydrophilic and hydrophobic surfaces was reattachment of lift-off bubble to heated surface, but bubble departure from nucleation sites was a good indication of OSV at hydrophobic surface.