Abstract: The two-phase geyser flow instability under low pressure natural circulation condition was researched experimentally. Using the experimental data, the RELAP5 code for the simulation geyser flow instability was also verified. The experimental results show that the period variety of effective driving pressure head induces the occurring of flow oscillation phenomena. The higher the fluid temperature in the downcomer is, the shorter the fluctuation period is. When the fluid temperature reaches saturation value, the fluctuation period keeps constant. Compared with the experimental data, the calculated magnitude and frequency of the temperature and flow rate oscillations reproduced well. Therefore, the RELAP5/MOD3.2 software is able to model the geyser flow instability under low pressure natural circulation condition.