Abstract: The activity of dead burned magnesia is closely related to the properties of the new bio material magnesium phosphate cement (MPC). In this paper, the activity with respect to different species of magnesia has been quantified by the different methods of specific surface area, iodine number, content of activation MgO and the rate of reaction with citric acid. The relationship among the activity of dead burned magnesia, the setting time, compressive strength and the maximum temperature of MPC has also been studied. The results show that the methods of specific surface area, iodine number and the content of activation MgO can not be used to quantify the activity of dead burned magnesia, but the rate of reaction with citric acid is available for measuring the dead burned MgO activity.