Abstract: The experimental investigation of gas-liquid two-phase flow in a vertical channel with a 7×7 rod bundle was carried out under atmospheric and room temperature conditions, and the ranges of gas and liquid superficial velocities were 0.04-14 m/s and 0.238-1.860 m/s. The flow patterns were observed in the experiments with high-speed camera. Four distinct flow patterns including bubbly, bubbly-churn, churn and annular-churn were observed and founding churn flow was the main flow pattern. The boundary of the Hewitt’s flow pattern figure was amended. According to the experiment data, the friction pressure drop change in bubble flow was opposite to that in churn flow. According to the experimental data and predecessors’ research, the boundary of flow pattern transition from bubbly to churn in rod bundle channel was got.