The indentation behavior (Vickers) of Single‐crystal MgO was studied as a function of temperature (20° to 1000°C). Indentations were made on the {110} plane, with the indents oriented such that one indent diagonal was parallel to the 〈001〉 direction. Using etchant techniques, the dislocation etch pit structures were examined both in the plane of the indentation and in cross section. All the observed slip traces were found to be consistent with primary slip ({110}〈110〉), with no evidence of secondary slip, even at 1000°C. Radial cracking was observed only at the pair of indent corners joined by the indent diagonal parallel to 〈001〉. The crack length increased with temperature (T) for indentations conducted atT<800°C. For indents made at 800°C or higher, however, no cracking occurred. These results are discussed both with respect to an existing slip‐induced crack nucleation model, and the change in crack driving force and toughness with ind