Sintered magnesia clinker (also called sintered MgO) was prepared with Mg(OH)2 (prepared by light calcining hydration of magnesite) and magnesite as starting materials, respectively, by the technical process: light calcining (850 ℃ 1 h) → grinding → molding → firing (1 600 ℃ 3 h). The morphology and structure of light calcined MgO powders prepared with magnesite or Mg(OH)2 were analyzed by XRD, SEM and FT-IR. The sinterability and microstructure of sintered magnesia prepared with magnesite or Mg(OH)2 were researched. The results showed that: (1) light calcined MgO powders prepared with magnesite or Mg(OH)2 were both face centered cubic structure. The light calcined MgO prepared with magnesite had the residual of functional group CO32-, and that prepared with Mg(OH)2 didn’t have CO32-, and was easy to break, which reduced the grinding time. (2) Because of the high activity of the light calcined MgO prepared with Mg(OH)2 and the catalysis of the water residual in the decomposing process of Mg(OH)2, the apparent porosity of sintered magnesia decreased to 0.6% from 2.0% of before hydration, closed porosity decreased to 2.8% from 6.1%, and bulk density increased to 3.46 g·cm-3 from 3.29 g·cm-3.