The behavior of near-wall bubbles in subcooled flow boiling has been investigated photographically to identify the physical mechanisms of critical heat flux at subcooled and low-quality conditions. Visualization experiments were performed for water flow in vertical rectangular channels under atmospheric pressure for mass fluxes below 2020 kg/㎡s. The thickness and other features of the near-wall bubble layer were examined with the aid of a high-speed camera, a still camera and an 8 mm-camera recorder. The number of activated nucleation sites increased as the wall heat flux was increased. At sufficiently high heat flux about 5-7.5 MW/㎡, the appearance of vapor clot or blanket on the heated surface made a role of an obstacle between main liquid region and the region near heater. At such high heat flux, three characteristic regions were observed in the heated channel: (a) a superheated liquid layer with attached bubbles, (b) a flowing bubble layer consisting of large coalesced bubbles over the attached bubbles, and finally (c) the liquid core.