The structural, mechanical, thermal, electronic and optical properties of inorganic Pb-free double perovskites 26 (X = Cl, Br, I) compounds have been investigated using the first-principles with full-potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory, and the generalized gradient approximation (GGA-PBE). The computation of the lattice constants showed a good agreement between our results and the experimental results and the theoretical data available. Using the three independent elastic constants 11 , 12 and 44 , we obtained various mechanical and thermal properties such as: bulk modulus , shear modulus , Young’s modulus , Pough’s ratio /, Frantesvich ratio /, Poisson’s ratio , Shear anisotropy character and Debye temperature for 26, 26 and 26 respectively. Our calculated elastic constant results show that 26, 26 and 26 are mechanically stable and anisotropic and ductile in nature. Furthermore, the 26 compound has the highest Debye temperature, which indicates the highest melting temperature. The electronic band structures reveal the semiconductor conductivity of the three compounds, with a band gap suitable for photovoltaic applications, in the range of 1-1.56. Finally, 26 (X = Cl, Br, I) HPs compounds exhibit excellent optical properties such as the optical absorption, conductivity and refraction index.