Due to the increasing consumption of energy and the green house effect caused by CO2 nuclear power stations have been playing more important role than ever in the electric power generation. And both the economy and the safety of the nuclear power station are strongly demanded. One of the restriction in the thermal design of the safe and economic nuclear reactor, especially in BWR is pointed out to be the heat removal from the fuel rods near the rods supporting spacer. The purpose of the present paper is to investigate the mechanism of the water film breakdown which results in the burnout of heating tube near the spacer under various flow pattern. The experimental results show that the film breakdown occurs near the leading edge, i.e., just below the spacer, as well as the inside of the down parr of the gap between the heating tube and the spacer.