In order to know the temperature gradient and heat flow in boiling heat transfer, Schlieren photograph technique is used and some of the observed results are shown here. The main features that have been found are ; (1) at low heat flux, each bubble is followed by a beautiful hot liquid tail ; (2) bubbles are ejected into liquid regardless of the direction of gravity by the inertia force of motion of the surrounding liquid ; (3) at high heat flux, a secondary "bubble cloud"covers the original bubbles which become smaller and greater in number and the bubble tails become smaller or vanish ; (4) transition boiling has much similarity to nucleate boiling and the former would be explained as an extension of the latter into the region where the heat flux shows a decrease in spite of an increase of wall temperature because of the decrease of the area of the liquid thermal boundary layer.