Abstract: High density magnesia clinker characterized by having a chemical composition, as oxides in weight %, ofMgO: 97.5% or moreCaO: 0.8-2.0%SiO.sub.2 : 0.12-1.0%Fe.sub.2 O.sub.3 : 0.3% or lessAl.sub.2 O.sub.3 : 0.15% or lessB.sub.2 O.sub.3 : 0.03% or lesswith a bulk density of at least 3.48 g/cm.sup.3. The said high density magnesia clinker can be produced by forming magnesium oxide by calcination of naturally occurring magnesite having a composition, based on ignition in weight %,MgO: 97.5% or moreCaO: 0.8-2.0%SiO.sub.2 : traces to 0.15%Fe.sub.2 O.sub.3 : 0.3% or lessAl.sub.2 O.sub.3 : 0.15% or lessB.sub.2 O.sub.3 : 0.1% or less,grinding the same after, or with, addition of amorphous silica, silica gel or a silicon compound capable of forming SiO.sub.2 by dead-burning in such a manner as to have the SiO.sub.2 content of 0.12-1.0% after dead-burning and dead-burning.