Abstract: A lightweight magnesia clinker containing magnesium oxide in an amount of more than 85 wt. % and having an apparent porosity of more than 40 vol. % and a bulk specific gravity of less than 2.0 g/cm.sup.3 is disclosed. In the lightweight magnesia clinker, the amount of magnesia clinker particles having a particle diameter of smaller than 3 mm is more than 90 wt. % of the amount of all magnesia clinker particles, and the amount of pores formed in the magnesia clicker particles having a pore size of smaller than 50 .mu.m is more than 90 vol. % of the amount of all pores. A process for the preparation of said lightweight magnesia clinker is also disclosed.