Effect　of　impurities　in　magnesia　on　reaction　between　magnesia　clinker　and　carbon　was　basically　studied　using　a　simphfied　system．The　reaction　was　estimated　by　measurement　of　weight　loss　of　magnesia　specimens　reacted　with　graphite　at　1500－1750℃． 　　The　results　obtained　are　summarized　as　follows； （1）　Reaction　between　magnesia　clinker　and　carbon　was　affected　chemical　composition　of　magnesia　clinker　rather　than　crystal　size　of　it．Therefore　in　high　purity　system　no　difference　was　observed　in　reactivity　of　sintered　and　fused　magnesia　with　carbon． （2）　B2O3　in　magnesia　clinker　accelerated　the　reaction　remarkably．This　was　attributed　to　liquid　phase　in　CaO－B2O3　system　formed　at　high　temperature，which　promoted　mass　transfer． （3）CaO　in　magnesia　clinker　accelerated　the　reaction，on　the　other　hand　Al203　depressed　it．Weight　loss　of　magnesia　specimens　after　the　reaction　at　1600℃　with　involved　CaO　and　Al2O3　respectively　corresponded　to　lattice　constant　of　these　specimens　quenched　from　1600℃．Then　reactivity　between　magnesia　　clinker　and　carbon　was　explained　from　the　standpoint　of　crystallography．

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