Effects of gap sizes(3.9 and 15mm) of vertical annuli and the bottom blockage on the nucleate pool boiling heat transfer at atmospheric pressure condition have been examined experimentally, and the results were compared to those with a single tube without confinement. The annular geometry resulted in significant increase in heat transfer coefficient. The effect is much enhanced with the bottom blockage. The heat transfer coefficient for the closed bottom condition is three times greater than the unconfined tube at 30kW/㎡ when the gap size is 3.9mm. However, with further increase of the heat flux much more than 70kW/㎡, all these effects were diminished.