In order to investigate the effect of particle size on expansive behavior of MgO-based expansive agent (MEA), linear expansion of paste containing MEA and hydration process of MEA in paste were measured. The results indicated the expansion of paste increased with the drop of particle size at early age, however, MEA with lager particle size expanded faster at late age. Finally, the expansion of paste increased with the rise of particle size. MEA with larger particle size was detrimental to the soundness of cementitious matarials. These two rules were more obvious to MEA burned at higher temperature. In addition, the hydration degree of MEA increased with the drop of particle size.