Prismatic dislocation loopsb= 1/2⟨101⟩, were observed around impurity precipitates in as‐grown magnesium oxide single crystals with a high percentage of Ca and Al impurities. The loops lay on {101} slip planes. By using the results of the dynamical theory of electron diffraction including absorption and stereo microscopy, the majority of the loops were identified asb= 1/2⟨101⟩ vacancy type. The possible chemical form of these impurities is discussed in view of the nature of the dislocation loops.