Boiling heat transfer experiments were conducted to examine dry-out phenomena in a two-phase natural circulation system of R-113. This work followed closed two-phase thermosiphon experiments and was performed with the same heating sections. The dry-out heat fluxes were much higher than those of the closed thermosiphons. The results showed that the dry-out heat flux rose as the circulation flow rate increased, and was little affected by the heating surface configuration. Dry-out was observed near the exit end of the heating section under an annular flow state. The relationship between the dry-out heat flux and the liquid film flow rate was analyzed and a correlation to predict the dry-out heat flux was proposed based upon the present results and data obtained using a forced upflow boiling system.