A new experimental attempt was made to simultaneously observe the dynamic behaviors of bubbles and dry spots in the vicinity of boiling surface. Also, the two-dimensional bubble structures Were obtained separately. From the visualization results, the formation of bubbles and dry spots occurs simultaneously. At critical heat flux (CHF), the surface rewetting is repeated by the local nucleate boiling around the larger vapor film. At just after CHF, nucleate boiling at the locally wetted region is extinguished, resulting in the dryout of the whole heater surface. Therefore, we conclude that CHF is initiated, from the locally limited nucleate boiling activity. rather than any hydrodynamic instability. (C) 2003 Elsevier Science Ltd. All rights reserved.