Abstract: A MgO-based expansive agent has been prepared by calcined magnesite under 1000℃ in the laboratory. The influences of MgO dosage, curring temperature and curring methods on the deformation behavior of cement pastes with this expansive agent were systematically studied. Meanwhile the effect of fly ash on the expansion ability of MgO was discussed. The results showed that the expansion produced by MgO in cement pastes is stable through the whole course of hydration, and with increasing the curring temperature, the expansion is enlarged. The cement paste containing 6% MgO exhibits autogenous expansion at 40℃ and tends to be stable after 90d. And the expansion performance of MgO was restrained by the addition of fly ash obviously.