This chapter addresses characteristics of flashing-induced density wave oscillations on the basis of the experimental results in a boiling natural circulation system with an adiabatic chimney. Flashing is caused by the sudden increase of vapor generation due to the reduction in hydrostatic head, since saturation enthalpy changes with pressure. Flashing-induced density wave oscillations may, therefore, occur at low pressure. The oscillation period correlates well with the passing time of bubbles in the chimney section regardless of the system pressure, the heat flux, and the inlet subcooling. According to the stability map, the flow became stable below a certain heat flux regardless of the channel inlet subcooling. The stable region enlarged with increasing system pressure. Therefore, the stability margin becomes larger by pressurizing the loop sufficiently before heating.