The invention provides a polycrystal magnesium oxide (MgO) sintered body which is capable of having a sintered density close to a theoretical density thereof. The MgO sintered body exhibits excellent mechanical properties and heat conductivity, while reducing contamination of an atmosphere due to gas generation. The invention also provides a production method for the sintered body. The polycrystal MgO sintered body has a unique crystalline anisotropy in which (111) faces are oriented along a surface applied with a uniaxial pressure at a high rate. The polycrystalline MgO sintered body is obtained by a method which includes the steps of: sintering an MgO raw material powder, having a particle size of 1 μm or less, under a uniaxial pressure and then subjecting the sintered powder to a heat treatment under an atmosphere containing 0.05 volume % or more of oxygen, at a temperature of 1273 K or more for 1 minute or more.