The effects of magnesioferrite precipitates on the mechanical properties of MgO single crystals were determined in samples with Fe/(Fe+Mg) ratios of 0.14 to 3.65%. Microhardness, slip‐band length, and 4‐point bending and compression strengths were measured. Maximum strength occurs for very short aging times, e.g. after 7° min at 700°C for samples with 3.65 cation % Fe, the hardness and critical resolved shear strength were 2300 and 35 kg/mm2, respectively. On further aging, the strength decreases rapidly at first from the peak value and then decreases slowly. Particles with diameters of ∼40 Å give maximum strength; fracture stresses>100,000 psi were observed. The strengthening is attributed to stacking faults in the particles and particle‐matrix surface area created when dislocations cut particles. The sharp peak in the strength vs aging curve is attributed to the particle‐matrix surface area and the main hardening to stacking faults. The critical resolved shear strength values are related quantitatively to the volume fraction and size of the precipitates in the theoretical treatme