In case of severe accident when core melt is ejected from vessel, external core catcher has been considered in APR1400 reactor. Two-phasenatural circulation basedcore catcher cooling system removes decay heat of corium to increases the reliability and safety. KAERI (Korea Atomic Energy Research Institute) have developed core catcher concept which operates as a passive system. As coolant flows through single channel below the core catcher plate, bubbles are generated due to decay heat and a two-phase flow is established.In order to stabilize the corium effectively and avoid critical heat flux (CHF) at the plate surface, it is necessary to understand two-phase flow behavior in the cooling channel.