Abstract: A series of natural circulation experiments were carried out in order to explore the flow instability mechanism. The flow boiling mechanism and local heat transfer characteristics were analyzed for different natural circulation oscillation modes. The experimental results show that the periodic and regular oscillation is induced by flow boiling under low and/or medium heat flux. Meanwhile, the wall superheat varies within a small range when flow rate oscillates. Under high heat flux conditions, an irregular but periodic oscillation is observed in the experiment. The transition of different boiling regimes is not the unique reason for this oscillatory circulation mode. Large amplitude oscillation is likely to trigger transient critical boiling. Periodic dryout occurs near the outlet and gives rise to the decrease of local heat transfer coefficient and the fly up of wall temperature. With the increase of heat flux, continuous dryout might take place in natural circulation system.