An experimental investigation of the natural circulating boiling flow characteristic in three cases of annulus with different outer diameter, and the effect of annular gap size on the burnout behavior is presented. The experimental work was conducted for each case of test section at system pressure of $1kg/cm^2$ and inlet subcooling $0-20^{circ}C$ in the full range of throttling ratio. As the result, the following facts were found. 1) With the increase of ${Delta}T_{sub}{;},D_{2}$ and A/A_{o},{;}q_{BO}$ increases on the whole, and with the decrease of ${Delta}T_{sub}$ and $D_{2}$, hydrodynamic instability is accelerated to happen prematually. 2)With the increase of ${Delta}T_{sub},D_{2}$ and A/A_{o}$ burnout characteristic shows the high velocitylow quality burnout, and with the decrease, low-velocity-high quality burnout. 3)With the decrease of A/A_{o}$, hyddrodynamic instability is singnificantly restrained and the difference of $q_{BO}$ in each $D_2$ under same condition is gradually reduced, finally converging into $1.9{ imes}10^{5}kcal/m^{2}-hr$