Abstract: The void fraction on the pressure-balance-line (PBL), which connecting core makes tank (CMT) and cold leg in AP1000, determines the injection of CMT. The steam in the PBL comes from the entrainment of cold leg. To study the effect of horizontal stratification entrainment on the characteristics of AP1000 passive core cooling system, a numerical modeling of AP1000 using the best estimate code Relap5/Mod3.2 was developed. The small break loss of accident (SBLOCA) was simulated. Comparisons of the results with and without entrainment model show that existing horizontal stratification entrainment model of Relap5 is very sensitive and important to the short-term of LOCA, and has significant impact on the entire SBLOCA process.