Abstract: Visualization technique was adopted to study the gas bubble behaviors in the vicinity of mixing vane of spacer grid in a 3×3 rod bundle under ambient temperature and atmospheric pressure. It was found that the bubble stagnation, which is dependent on liquid flow rate and volume fraction, occurs at the leeward of the mixing vane. The stagnation bubble interface oscillated with flow and inflow bubble coalescence. Smaller bubbles were generated by liquid entrainment at the end of the stagnation bubble, which is one of the main mechanisms of bubble size redistribution. The stagnation bubble size increased and break-up bubble size decreased with the increasing of the liquid flow rate under identical void fraction. The stagnation bubble size was independent upon void fraction under identical liquid flow rate, while the oscillation of stagnation bubble interface was strongly influenced by the void fraction, especially the inflow bubble size. The bubbles, passed through the mixing vane from windward and leeward mixing vane, led to different bubble flow directions and phase distribution at the downstream of the spacer grid.