Abstract: The flow instability in natural circulation system with single channel was performed by the best estimate system computer code RELAP5. The instability boundaries of different pressures were obtained. The influences of system pressure and the riser section on the flow instability were analyzed. And the characteristics of oscillation curves were studied. The results confirm that the subcooling boiling is very important for the natural circulation instability. The investigation results have the values for the design of some natural circulation system.