The application of magnesia-based construction materials draws much research interests nowadays due to the ever increasing awareness of environmental protection. By incorporation of fly ash into magnesium oxychloride (MOC) cement, an energy efficient and environmentally friendly repair material can be formed for successful industrial applications. In the current research, an appropriate formulation of the MOC matrix with a suitable combination of the molar ratios MgO/MgCl and HO/MgCl has been characterized by using phase diagram, X-ray diffractograms and scanning electronic microscope. Subsequently the influences of fly ash on the properties of both MOC cement and mortar are investigated. It is found that the incorporation of fly ash can enhance the workability or fluidity, retard the setting time, and improve the water resistance of the MOC mortars. With the enhanced performance and a slightly expansive nature, the MOC mortars incorporated with fly ash has a good potential to be used as a repairing material. © 2009.