The surface structures of a Ba/Mo(110) system have been investigated by reflection high-energy electron diffraction (RHEED). The characteristics of three surface structures, namely, the cð2Â2Þ, (1:6Â2), and (1:5Â1:8) structures, were already reported. In particular, the RHEED patterns due to the cð2Â2Þ structure have characteristic spots with small elongations, but the origin of such a feature and the characteristic aspect of atomic arrangements of the cð2Â2Þ structure have not been discussed up to now. In this paper, a new interpretation for the elongations of a RHEED pattern due to the cð2Â2Þ structure of the Ba/Mo(110) system is reported.