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．
