<p>The structural characteristics, elastic properties, Debye temperatures, thermal conductivities, and damage tolerances of RuO₂ with different crystal structures (cubic RuO₂-C and tetragonal RuO₂-T) have been studied by means of first-principles calculations based on density functional theory (DFT). From the data obtained, RuO₂-C and RuO₂-T have thermodynamic stability and the dynamic stability of both phases is confirmed by the absence of imaginary frequencies in their phonon spectra. Furthermore, it can be seen that the deformation resistance of cubic structure (RuO₂-C) is greater than that of tetragonal structure (RuO₂-T); that is, cubic structure (RuO₂-C) has better stiffness and is not easy to deform. Meanwhile, cubic structure (RuO₂-C) has lower toughness, higher hardness, and weaker damage tolerance. In the aspect of elastic properties, 3D surface’s structure diagram and 2D plane projection diagram of Young’s modulus and shear modulus varying with direction are drawn to better visualize elastic anisotropy, which shows that the elastic anisotropy of tetragonal structure (RuO₂-T) is relatively large. In addition, cubic structure (RuO₂-C) has larger Debye temperature, thermal conductivity, and smaller thermal expansion coefficient. This work provides the comprehensive first-principles comparison of the anisotropic elastic and thermal properties of cubic and tetragonal RuO₂, offering fundamental insights for their potential applications in extreme environments.</p>