Abstract: In the boiling heat transfer setup,the two commonly methods to heat the test section wall are electric heating and fluid heating,and the boiling heat transfer for the latter is seldom studied.The boiling heat transfer characteristics under the two heat methods are different.Visualization research has been conducted on the bubble behavior of sub-cooled flow boiling water in vertical annular ducts heated by water.The duct widths are 5mm and 3mm,and the mass flux ranges are 16.8～55.3 kg/（m².s） and 15.3～62 kg/（m².s）,respectively.The results show that there exists a bubble layer near the wall,and most bubbles move in the layer.Most bubbles in 3mm annulus will slide for a distance not more than 2～3 times of the bubble diameter and keep expanding and shrinking for several times before departure.On the contrary,most bubbles in 5mm duct scarcely slide and are always bounced to the wall after departure.All these bubble behavior should be one of the reasons that the boiling can enhance heat transfer greatly.