The phenomena of natural-convection boiling and critical heat flux; on the outer surface of a heated hemispherical vessel surrounded by a thermal insulation structure were investigated experimentally. The objectives were to observe the behavior of the boiling-induced hue-phase motion in the annular channel formed between the hemispherical vessel and the insulation structure and to determine the flow effect on the natural-convection boiling process on the downward-facing hemispherical surface. High-speed photographic records revealed the presence of violent cyclic ejection of the vapor masses generated by boiling on the vessel outer surface, which resulted in a buoyancy-driven, upward, co-current two-phase flow through the channel. When boiling was taking place at high heat flux levels, the flow through the minimum gap of the channel was found to be highly unsteady and chaotic. Measurements of the local boiling heat fluxes and the local wall superheats were made which showed a significant spatial variation of the nucleate boiling heat transfer. Owing to the effect of Be unsteady and chaotic two-phase motion, the local critical heat flux had the lowest value near the minimum gap.