Experiments were conducted to study the flow and heat transfer characteristics of a natural circulation cooling system for electronic components. It consisted of a test section, a horizontal evaporator, a vertical tube, a horizontal condenser, a rubber bag attached at the exit of condenser, a downcomer, a mass flow meter, and a liquid subcooler. FC-72 was filled in the closed loop. In the test section, a silicon chip with dimensions of 10×10×0.5 mm3 was fixed at the bottom of a horizontal duct with dimensions of 5×14 mm2. A smooth chip and two chips with square micro-pin-fins were tested. The flow rate of FC-72 was correlated well as a function of static head difference between the vertical tubes ΔΡ. The flow became unstable below a certain value of ΔΡ. The heat transfer results were compared with those for pool boiling. All chips showed a boiling curve similar to the case of pool boiling except that the critical heat flux was 5-18% smaller than pool boiling.