The　behavior　of　slag　penetration　into　MgO　refractory　was　investigated　by　combiningi n-situ X-ray　observation　with　microstructural　analysis　of　the　samples　after　penetration　experiments．The following　results　are　obtained： The　slag　penetrates　rapidly　into　the　refractory，reacting　with　MgO particles　in　the　refractory．The　slag　penetrates　unevenly　into　the　refractory　with　uneven　pore　size in　a　path　like　a　tree，firstly　along　the　main　path（large　pore），and　then　extending　to　the　branch　path （small　pore），while　evenly　into　the　refractory　with　even　pore　size．The　rate　of　slag　penetration increases　with　increasing　pore　radius　of　the　refractory，T．Fe　concentration　in　the　slag　and experimental　temperature，and　with　decreasing　the　slag　basicity （C／S　ratio）．In　the　case　of　the 10 mass％ Al203－bearing　slag，the　rate　of　slag　penetration　is　less　than　that　of　the　Al203－free　slag　in　the initial　stage．The　penetration　of　the　10 mass％Al203－bearing　slag　into　the　sample　with　the　apparent porosity　of 31 ％ stopped　after　reaching23mm　in　height　under　the　present　experimental　 conditions， which　was　much　smaller　than　that　of　the　Al203－free　slag．In　the　initial　stage　of　penetration，the penetration　height　is　proportional　to　the　square　root　of　penetration　time，and　the　penetration behaviors　of　the　slag　can　be　qualitatively　explained　based　on　a　capillary　model．　The　penetration　of　the　Al203－bearing　slag　is　remarkably　suppressed　by　the　formation　of　MgO－FeO－Al203 spinel　caused by　the　reaction　of　Al203 and　FeO　in　the　penetrated　slag　with　MgO　particles　and　MgO　dissolved from　the　refractory．

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