The Karlsruhe ceramic breeder blanket design for a demo reactor and for the test objects in NET is based on spherical lithium-orthosilieate pebbles of 0.35-0.6 mm diameter. This paper deals with the structure and mechanical properties of pebbles and their improvement by heat treatment. The pebbles are fabricated from molten lithium-orthosilicate at Schott Glaswerke, Mainz. This method provides almost spherical shapes and a high packing density. The production process involves rapid quenching of the liquid droplets from the melt to room temperature which causes internal stresses and leads in some cases to formation of microcracks and a scattering of mechanical properties. Release of stresses is achieved by heating at temperatures above 700 ยฐ C with subsequent slow cooling. Healing of microcracks, however, can only be attained by heating to temperatures above the melting point of the Si-rich phase ( > 1024 ยฐ C) if the material has a slight surplus of Si over the stoichiometric orthosilicate composition. It results that heat treatment of pebbles for a short time at 1030 ยฐC improves the mechanical stability. The change in structure of the material during heat treatment is examined by ceramography of particle cross sections.