The influence of CaO on the mechanical behaviour of translucent, MgO-doped Al2O3 ceramics has been investigated, using alumina both without and with added CaO. The addition of only 40 wt ppm CaO drastically changed the microstructure, resulting in an increase of optical transmission, which was the purpose of the addition. Strength (∼300 MPa) and fracture toughness (∼3.7 MPa m1/2), however, as measured at room temperature, decreased by 30% and 10%, respectively, while the resistance to slow crack growth increased slightly. Toughness as well as strength, showed a relative decrease with temperature of about 3×10-4 Ks-1. The results are discussed and compared with available literature data.