Effect of starting particle size on hot-pressing of magnesium oxide (MgO) powder 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 (compacts) were hot-pressed at a temperature between 900 and 1300 degrees C. The densifications of these compacts during the hot-pressing proceeded via (i) the sintering of primary particles within secondary particles and the rearrangement of secondary particles/grains (900 degrees C), (ii) the gradual grain growth controlled by the pore migration (900 similar to 1100 degrees C) and (iii) the rapid grain growth due to the active mass transfer (1300 degrees C); the grain sizes of MgO compacts hot-pressed at and below 1100 degrees C were < 1 mu m, while those at 1300 degrees C attained 20 similar to 30 mu m. The translucent compact with the relative density of 99.7% could be obtained when the compressed powder with the average primary particle size of 44 nm was hot-pressed at a temperature as low as 1100 degrees C for 1 h.