The atomic structure of a MnO-deposited Rh(100) surface was studied using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The STM image of MnO/Rh(100) at a Mn coverage of 0.80 showed two hexagonal domains rotated by 90 degrees with respect to each other. One of the unit vectors for these hexagonal domains had the same direction as the unit vector of the Rh(100) substrate, suggesting that these hexagonal domains grow in the [011] or [011] direction of Rh(100). An averaged spacing of 3.1 Angstrom was obtained by the STM for the nearest-neighbor atoms of the hexagonal layer. The LEED pattern supported the presence of two hexagonal domains rotated by 90 degrees by 12 extra spots in addition to the spots of the Rh(100) substrate. A lattice constant of 3.1 Angstrom for the hexagonal domain measured by LEED coincided with that measured by STM. The hexagonal layer was found to be stable at 800 K. Although the Mn/Rh(100) surface showed no hexagonal LEED pattern or hexagonal STM image, the oxidation of the Mn/Rh(100) surface resulted in the appearance of a hexagonal pattern identical to those observed for the MnO/Rh(100) surfaces. The stable hexagonal domains were thus attributed to the monolayer of MnO species covering the Rh(100) surface.