Abstract: The ADS-4 depressurization and entrainment test loop (ADETEL) scaled after AP1000 was constructed to investigate the tee branch entrainment process. The experiment which consists of the onset of entrainment and entrainment rate was accomplished under the double gas inlets conditions. The entrainment process was recorded by a high speed camera and analyzed in detail. The experimental data were obtained and compared with existing data and correlations. The results show that relatively large discrepancies exist in the comparison due to the differences of test section geometric configuration and test condition. The entrainment frequency was investigated in the experiment. It indicates that the entrainment period declines dramatically with the increase of entrainment rate at the low entrainment rate. At the high entrainment rate, the entrainment period gradually stabilizes.