Single crystals of the magnesium niobium oxide Mg4Nb2O9 were prepared at 1400 degrees C. This compound has the ordered corundum structure with the space group P (3) over bar c1 (No. 165) and the lattice parameters are a = 5.1612(7) and c = 14.028(1) Angstrom. The final R-factors were R = 0.032 and R-w = 0.031 for 1092 unique reflections. The face-shared octahedra are occupied by the same cations and the cation is ordered in the way of an alternative stacking of two layers of a mixture of Mg and Nb (1:1) and one layer of Mg along the c axis.