[en] The flexure strength and the intensity of the photon emission during fracture were measured to investigate a correlation between them on single crystal and polycrystalline MgO. Prior to the fracture, IR and UV/vis photons were emitted due to the plastic deformation only in the single crystal. At the moment of the fracture, intense IR and UV/vis photon emissions were observed in both of the single crystal and polycrystalline MgO. In the single crystal, the peak emission intensities of the UV/vis and IR photons were positively correlated with the flexure strength. In the polycrystalline MgO, there was no clear correlation between the flexure strength and the UV/vis photon emission intensity, whereas the IR photon emission intensity was weakly and positively correlated with the flexure strength. The result suggests that crack propagation behavior would affect only the UV/vis photon emission during the fracture of MgO