A visualization study on boilihg phenomena at various surface orientations was conducted to investigate the bubble behaviors and the critical heat flux (CHF) mechanism. The vapor behavior observed in this study indicates that the pool boiling can be categorized into three regions: upward facing (0 degrees <= theta < 90 degrees), vertical and near-downward facing (90 degrees < theta < 165 degrees), and downward facing (165 degrees < theta < 180 degrees). In the upward-facing region, the vapor was generated and detached easily in the vertical direction of the heated surface. In the vertical and near-downward facing region, the vapor grew and drifted upward the surface in a wavy shape owing to the vapor flow. When the heater faced downward (165 degrees < theta < 180 degrees), the vapor was trapped and blanketed the entire heated surface. According to the vapor behaviors at various inclination angles, the CHF decreased with the increase of the surface orientation from horizontal facing upward to downward. The vapor film thickness decreased as the surface orientation increased from 0 degrees to 180 degrees. The vapor velocity affected the distance between the two wavelength peaks of the vapor. Faster vapor flow yielded a shorter wavelength. Additionally, the CHF location was affected by the surface orientation owing to the vapor behavior. These results indicate that the surface orientation significantly affects the vapor behavior, CHF, CHF location, vapor velocity, wavelength, and maximum and minimum vapor thicknesses on a printed circuit board in a saturated water pool. (C) 2018 Elsevier Ltd. All rights reserved.