The natural convection of water in the vertical narrow annular gaps is visualized by using black ink and temperature distributions of a inner pipe wall are measured. The following results are obtained:(1) A uniform temperature distribution along the circumferential direction remains unchanged until the Rayleigh number which defines the convection generation reaches the critical value.(2) When the critical Rayleigh number exceeds, a pair of large circulating flows becomes visible.(3) The gap diatance d influences the value of the critical Rayleigh number, but the flow behavior does not change.