Abstract: The test loop in the scaled ratio 1∶5.6 was constructed to repeat the process of upper plenum entrainment in AP1000. The upper plenum entrainment rate in different steam flow rates and different liquid levels of the pressure vessel was studied. The results indicate that the entrainment rate increases with the liquid level in the vessel and is affected by the steam flow rate weakly. The entrainment rate remains stable at lower liquid level. The upper plenum entrainment strengthens significantly with the insertion of the reactor internals.