Abstract: The onset entrainment and stable entrainment for T-junction composed of a large downward-oriented branch and a horizontal main pipe were researched based on visualization experiment. The gas entrainment experiment results were compared with the calculating results from RELAP5. The experimental results indicate that the formation of vortex at the onset entrainment is similar with the previous research results. A new vortex-free entrainment formation is observed during stable entrainment. The entrainment quantity of gas has linear relation to the pressure difference. The entrainment quantity of gas decreases with the increase of pressure difference between the branch and the horizontal main pipe.