In this study, nanoindentation coupled with scanning electron microscopy technique for the microstructural and micromechanical characterizations were applied on the magnesium oxychloride (MOC) cement paste. For the first time, micromechanical properties of the reaction product, phase 5 (5Mg··8O) in MOC cement system were investigated. It was determined that the average indentation modulus and hardness of phase 5 were 32.5 ± 4.2 GPa and 1.03 ± 0.19 GPa, respectively. Moreover, the elastic modulus of the MOC cement paste at micro- and macro-scales were further correlated, which built a framework for predicting its macroscopic elastic modulus.