Abstract: With the T-junction formed by the fourth stage automatic depressurization system (ADS-4) and hot leg in AP1000 nuclear power plant as object of study, a small-scale experiment research on entrainment at T-junction was performed. The visualization research indicates that the entrainment phenomena through a large size branch are apparently different from those through a small branch. Two entrainment mechanisms could be found in the stratified flow entrainment area. Intermittent flow entrainment tends to occur at a lower Froude number and a lower gas chamber height. Annular flow entrainment always arises at a higher Froude number. Also, the experimental research shows that the reverse flow is observable in a large size branch inlet. The geometry of the branch has an important effect on the onset entrainment. In addition, liquid crossflow does not seem to affect the onset entrainment.