The surface structures and the growth mode of Mg deposited on Mo(1 1 0) surface have been investigated using a molecular beam epitaxy system with a reflection high-energy electron diffraction. The surface phase diagram of Mg on the Mo(1 1 0) substrate for deposition thickness versus substrate temperature was completed. Three kinds of surface structures, a, b, and c were observed in the range of 0.1 ML $ 4.0 ML. These surface structures are commensurate with the atomic arrangement of the Mo(1 1 0) plane. The dense direction of each structure is parallel to the [0 0 1]Mo orientation and the Mg atoms are commensurate every five, four and six Mo atoms in ½1 1 1Mo orientation in a, b and c structure, respectively. Moreover, the b structure has the same arrangement of a bcc(1 1 0) plane rotated by 54.7°from the Mo(1 1 0) plane with elongation by 1.17 times. The d structure, which is three-dimensional crystallites of Mg in initial stage of crystal growth, was observed at the thickness more than 4.0 ML below 200°C. From this pattern it was found that the crystallites of Mg have a specific epitaxial orientation relationship for the Mo(1 1 0) substrate, ½1 1 0 0Mgk½1 1 0Mo and (0 0 0 1)Mgk(1 1 0)Mo, and Mg film grows by Stranski-Krastanov growth mode.