Abstract: An experimental investigation on startup characteristics of open natural circulation system was performed. The results show that heating powers have significant influence on the flow characteristics evolvement during startup process. Namely, the system undergoes single-phase natural circulation, geysering oscillation and flashing oscillation under low power conditions; it undergoes single-phase natural circulation, geysering oscillation and boiling accompanied with flashing oscillation, and then develops into the steady circulation of two-phase flow under the medium power conditions, the system under the high power conditions undergoes the same evolvement process as that under the medium power conditions at beginning, and finally develops into density wave oscillation. The main reason leading to the evolution of flow characteristics during startup process is that the boiling in the heated tube continues to enhance, and the flashing phenomenon in the upflow-leg enhances at first, and then weakens, with the increase of the entrance temperature of the heated tube. The two factors interact with each other, which bring about the obvious changes of the system circulation flow rate, the phase transition positions and the void fraction. Finally, a non-dimensional flow instability map of the open natural circulation system during startup process was drawn. And empirical correlations of the instability boundaries for geysering oscillations and flashing dominated oscillations were established. The fitting results fairly well agree with experiment data.