RuAl2 shows the abnormal semiconductor property. However, the relationship between structure, electronic and mechanical properties of RuAl2 is not well understood. In this paper, we systematically investigate the structural stability, electronic and mechanical properties of RuAl2. Four possible RuAl2 structures (TiSi2-type, TiAl2-type, OsAl2-type and AuAl2-type) are estimated and predicted. We find that although TiSi2-type structure is more thermodynamic stability than that of other structures, the predicted structures are also stability at the ground state. Importantly, TiSi2-type structure shows a semiconductor property with a band gap of 0.189 eV. However, other three structures exhibit metallic behavior because of electronic overlap between A1-3p state and Ru-4d state at Fermi level. We further find that the predicted OsAl2-type structure has strong shear deformation resistance and high elastic stiffness in comparison to other three structures. The reason is that the bond strength of Ru-Al metallic bond for OsAl2-type structure is stronger than that of other structures. (C) 2017 Elsevier Ltd. All rights reserved.