Abstract: The bubble nucleation in a narrow channel under sub-cooled flow boiling was experimentally studied. The high speed digital camera was used to observe the nucleation and sliding of bubbles. Most of the bubbles in the experiments slide along the heated surface, but not lift off the surface, as it is usual in a conventional channel. The nucleation of bubble was different at higher and lower system pressure. The nucleation sites at lower pressure distribute uniformly along the heated surface. However, most of the nucleation sites at higher pressure concentrate near the onset of nucleation boiling. The discrepancy of the bubble nucleation sites distribution is mainly caused by the different bubble sliding characteristics at different system pressures.