Differrence　between　sintered　and　electro－fused　magnesia　on　resistance　to　slag　corrosion　was　studied．Experiments　were　carried　out　in　case　of　dissolution　of　magnesia　to　liquid　slag　and　penetration　of　slag　or　Fe　ion　to　magnesia．The　results　obtained　are　summarized　as　follows： 　（1） Dissolution　of　magnesia　to　the　slag　obeys　the　Noyes－Nernst　equation．Therefore　the　dissolution　is　a　diffusion-controlled　process．Under　this　condition　on　which　corrosion　takes　place　by　a　solution　process　difference　of　the　dissolution　to　the　slag　between　sintered　and　electro－fused　magnesia　is　not　observed． 　（2） The　C/S＝2　slag　has　higher　capability　of　dissolving　magnesia　than　the　C/S＝4　stag．This　is　attributed　to　high　solubility　of　magnesia　in　the　C/S＝2　slag　and　high　value　of　DL/δ　which　results　from　the　low　viscosity　of　the　C/S＝2　slag． 　（3） In　the　C/S＝4　slag　test　slag　penetration　into　the　grain　boundary　of　sintered　magnesia　is　observed．Therefore　differrence　between　sintered　and　electro－fused　magnesia　exists．On　the　other　hand，in　the　slag　tests　with　lower　C/S　ratio　Fe　ion　easily　diffuses　into　magnesia　crystal　and　diffusion　coeficients　of　Fe　ion　in　sintered　magnesia　are　higher　than　those　of　electro-fused　magnesia．

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　Sinterable　magnesia　powder　was　prepared　from　high　purity　magnesium　basic　carbonate　by　soda　ashmethod．When　this　compact　sintered　at　1600℃　for　10 min，the　fired　body　had　99．5％　theoretical　density．Thermal　shrinkage　of　the　compact　was　measured　at　constant　heating　rate　and　the　sintering　behavior　was　analyzed　using　the　kinetics　of　Bannister’s　equation　conceming　initial-stage　sintering，The　obtained　results　were　as　follows． （1）　The　initial-stage　sintering　would　be　due　to　a　mechanism　such　as　volume　diffusion． （2）　The　estimated　activation　enegry　was　about　70kcal/mol　which　was　measured　by　Oishi　and　Kingery，as　oxygen　volume　diffusion．

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