Abstract: The study of the interfacial behaviors of the annular flow in rod bundle channels can provide data and theoretical support for the accident treatment of PWR and the normal operation of the BWR power plant. In this study, wavy interface forms of annular flow in 3×3 rod bundle channels were recorded and analyzed for drawing detailed regimes of annular flow patterns, and the mechanism analysis was carried out for the formation of interfacial behaviors. The results show that there are seven forms of annular flow in the rod bundle channels, i.e. churn, non-wetting, smooth boundary, single-wave, bag-shaped disturbance-crest, ligament-shaped disturbance-crest and liquid-loss in ligament-shaped disturbance-crest annular flow. When the gas velocity is high and the liquid velocity is low, the singlewave flow occurs, of which the interface moves with troughs and crests at a certain upward velocity, and this phenomenon can be explained by the Kelvin-Helmholtz instability theory. When liquid flow rate increases or gas flow rate decreases, the interfacial instability of single-wave occurs, so the energy of the interface increases resulting in changing the annular flow regime into bag-shaped disturbance-crest or ligament-shaped disturbance-crest. When the energy exceeds 1.6 kPa²/Hz (the binding capacity of the bottom liquid film), the flow regime changes to be the liquid-loss annular flow.