The effects of the narrowed upside gap on nucleate pool boiling heat transfer in a vertical annulus were investigated experimentally. For the study, a stainless steel tube with a diameter of 25.4 mm and saturated water that kept an atmospheric condition were used. The ratio between the gaps measured at the upper and the lower regions of the annulus ranged from 0.18 to 1. Two different lengths of the modified gap also were investigated. The change in heat transfer due to the modified gap became evident as the gap ratio decreased and the length of the gap increased. As the gap ratio became less than 0.51, a significant decrease in heat transfer was observed compared to the plain annulus. The longer gap size resulted in an additional decrease in heat transfer. The major cause for the tendency was attributed to the formation of lumped bubbles around the upper region of the annulus followed by the increased flow friction between the fluid and the surface around the modified gap.