Abstract: The ADS-4 depressurization and entrainment test loop (ADETEL) scaled with AP1000 as the prototype was constructed to investigate the entrainment process at the ADS-4 tee branch line. The experimental phenomenon of the onset of entrainment and the entrainment rate were recorded by a high speed camera. The data were obtained and compared with the existing data and correlations. The results show that large discrepancy between ADETEL experimental data and available data exists. RELAP5 and ATLATS correlations can not estimate the entrainment quality in AP1000 accurately. The entrainment rate declines rapidly with the decrease of liquid level in hot leg when the liquid level is low. The onset of entrainment is more likely to happen in smaller d/D (diameter ratio of branch to hot leg) conditions. The ADS-4 entrainment rate in AP1000 is lower than that in AP600 at the same dimensionless hot leg liquid level.