Abstract: Boiling heat transfer is a highly effective mode of heat transfer. Based on the visually experimental platform for pool boiling designed by research group, taking the boiling heat transfer over micro-wires with the diameters of 0.1, 0.2 mm as objects, the states of bubbles over the micro-wire under sub-cooled condition were studied with a high speed camera and the electronic microscope. The influence of heat flux and sub-cooling degree on the departure diameter and growth time of the bubble over micro-wire were analyzed, respectively. The results show that four kinds of bubbles sweeping movements and jet flow in the growth process can be observed on the heated micro-wire. The bubble jet flow continues for 13 s on the heated wire with the diameter of 0.2 mm, when the heat flux is 3.35×10⁶ W/m² and mainstream temperature is at 60℃. And the multi jet phenomenon appears over the bubble on the heated wire with diameter of 0.1 mm under the condition of heat flux 4.53×10⁶ W/m² and mainstream temperature at 70℃. When the temperature of mainstream reaches 70℃, the bubble sweeps strongly on the wire, therefore, the bubble departure diameter and growth time are significantly reduced.