[en] Natural circulation BWR actively equipped with passive safety features has been promoted to solve the recent challenges in BWR nuclear power and safety technology. With regard to startup stage, various thermo-hydraulic instabilities might be induced due to an elimination of re-circulation pumps. A lot of studies have been made on the instabilities in evaporated system as well as in a reactor. In the instabilities, geysering accompanied with flow reversal phenomena has been investigated in a vertical closed loop, U-shaped closed loop, twin parallel channels, and so on. However, in twin parallel study the effect of non-heated length on geysering has not been sufficiently clarified. The objective of this research is to experimentally investigate the thermo-hydraulic instabilities, particular in geysering, with a natural circulation loop consisting of parallel boiling channels and the single connection channel, which simulates the basic flow around the reactor core in the system pressure range from atmospheric to 0.7 MPa. The parallel boiling channels are consisted of heated and non-heated section. The heated section forms annulus and heated from the inner wall. The input heat flux range of 0 up to 580 kW/m², and inlet subcooling temperatures of 5, 10, and 15 K respectively, are imposed in the experiments. In the parallel channels with non-heated risers, three types of thermo-hydraulic instabilities were detected in the following sequence, geysering, natural circulation oscillation, and density wave oscillation. Especially in geysering, it is induced due to rapid condensation in the non-heated risers and it is not be suppressed even at 0.7 MPa though it has a tendency to be suppressed with an increase in the system pressure. On the other hand, in the parallel channels without non-heated risers, sinusoidal oscillation similar to natural circulation oscillation has been detected, and geysering had never observed. The new findings are that the sinusoidal oscillation is induced because of the hydrostatic head fluctuation in the connection channel, where the flow regime is constantly slug flow. The oscillating period is well correlated with the sum of delay time for boiling and passing time of slug bubbles in the connection channel. From the facts described above, it is found that non-heated region in a channel box should be as shorter as possible to prevent geysering from occurring, and sinusoidal oscillation similar to natural circulation oscillation is induced in any configuration of parallel channels. (authors)