Relationship between powder properties and bending strengths of the sintered magnesium oxide (MgO) specimens was examined using seven kinds of MgO powders prepared by a vapour-phase oxidation process; the average primary particle sizes were 11, 25, 32, 44, 57, 107 and 261 nm. These compressed powders (specimens) were fired at 1600 or 1700 degrees C for 1 to 15 h. Although the densification behaviours of the specimens varied with the primary particle size of the starting powders, the relative densities of the specimens fired at 1700 degrees C for 5 h were all in the range of 97-98%. The relationships between bending strengths and grain sizes of these sintered specimens could be classified into two categories, according to the primary particle size of the starting powder : (i) at and below 32 nm and (ii) 44-261 nm. In range (i), the bending strengths of the sintered specimens were as low as similar to 120 MPa; the grain size was reduced from 50.7 to 35.8 mu m as the primary particle size decreased from 32 to 11 nm. In range (ii), as the primary particle size increased from 44 to 261 nm, the bending strength of the sintered specimen was enhanced from 162 to 183 MPa, while the grain size was reduced from 28.3 to 13.7 mu m.