Earthquake is one of the most serious phenomena for safety of a nuclear reactor in Japan. Therefore, structural safety of nuclear reactors has been studied and nuclear reactors ware contracted with structural safety for a big earthquake. However, it is not enough for safety operation of nuclear reactors because thermal-fluid safety is not confirmed under the earthquake. For instance, behavior of gas-liquid two-phase flow is unknown under the earthquake conditions. Especially, fluctuation of void faction is an important factor for the safety operation of the nuclear reactor. In the previous work, fluctuation of void faction in bubbly flow was studied experimentally and theoretically to investigate the stability of the bubbly flow. In such studies, flow rate or void fraction fluctuations were given to the steady bubbly flow. In case of the earthquake, the fluctuation is not only the flow rate, but also body forth on the two-phase flow and shear forth through the pipe wall. Interactions of gas and liquid through their interface also act on the behavior of the two-phase flow. The fluctuation of the void fraction is not clear for such complicated situation under the earthquake.Therefore, in this research projects, the behavior of gas-liquid two-phase flow is investigated experimentally and numerically in the series of study. In this study, to investigate effects of vibration on bubbly flow in the components and construct experimental database for validation, we performed visualization experiments of vertical bubbly flow in rectangular water tank on which sine wave vibration was applied. In this paper, results of preliminary visualization experiments were shown including data of positions of bubbles, shapes of bubbles and velocity of bubbles evaluated by the visualization techniques. And liquid velocity distribution around bubbles by the PIV measurement was also shown. In the results, bubble behaviors were affected by oscillation. And the cycle of the bubble inclination angle was almost same as the cycle of oscillation table velocity.Copyright © 2013 by ASME