Heat transfer characteristic in a confined space has been studied for vertical narrow annuli with one-side heating. Experiments are performed for stilled water at 101325 Pa or so for annuli with heating section of 0.9 m and gap widths of 0.75 mm. The testing section is heated from inner side. The flow rates change from 1.66667 × 10−5 m3/s to 5.83335 × 10−5 m3/s. Four boiling regimes are identified through visual observation. It appears that the boiling heat transfer in narrow channel possesses a better heat transfer rate. Based on the experimental result, the Chen correlation is modified by Bond number to be applicable to the area of narrow space boiling heat transfer.