Cubic 8 mol% Y2O3-stabilised ZrO2 is well known for its good oxygen conductivity and therefore it is used in fuel cells, as oxygen sensor for liquid metals and as pH sensor in high temperature water. As a part of its fracture toughness assessment, the crack system caused by Vickers hardness indentations was examined by breaking specimens along an indentation diagonal. It was found that, although the normally observed fracture mode is transgranular, intergranular fracture occurred in the zone below an indentation. Annealing the specimen above 900 C before breaking caused the disappearance of this effect. The intergranular fracture mode was explained by the combined effect of plastic deformation under the indentation and environmental humidity. Annealing is expected to cause restoration to the un-deformed structure. Observations on the intergranular and transgranular cracking pattern, slow crack growth occurrence and step formation at the fracture surface, are reported.