The effect of the addition of magnesium (up to 10 wt%, expressed as MgO) on the phase composition, the microstructure and the mechanical properties of hydroxyapatite-based ceramics (HA) was studied by X-ray powder diffraction, infrared spectroscopy, scanning electron microscopy, and mechanical testing. Doping with magnesium did not change the crystal lattice of HA but, due to an isomorphous substitution of Mg 2+ for Ca 2+ , it caused a decrease in the average grain size, an increase of microporosity, a removal of the texture, and the formation of a weak intergranular boundary. The transcrystallite fracture pattern of pure HAwas changed to a predominantly intercrystallite one. This resulted in a decrease in density (down to 2.93 g cm -3), in microhardness (down to 400 MPa), in compressive strength (down to 190 MPa) and in improved plastic properties of the material (increasing the fracture toughness K 1C up to 2.7 MPa m 1/2 , close to that of the bone). It is interesting that the K 1C increase could be only observed for a small concentration of additives (up to 1 wt%), i.e. close to the Mg concentration in natural bone mineral.