This paper investigates the structural and electronic properties of rhenium diboride by first-principles calculationbased on density functional theory.The obtained results show that the calculated equilibrium structural parameters ofReB2 are in excellent agreement with experimental values.The calculated bulk modulus is 361 GPa in comparison withthat of the experiment.The compressibility of ReB2 is lower than that of well-known OsB2.The anisotropy of the bulkmodulus is confirmed by c/a ratio as a function of pressure curve and the bulk modulus along different axes along withthe electron density distribution.The high bulk modulus is attributed to the strong covalent bond between Re-d andB-p orbitals and the wider pseudogap near the Fermi level,which could be deduced from both electron charge densitydistribution and density of states.The band structure and density of states of ReB2 exhibit that this material presentsmetallic behavior.The good metallicity and ultra-incompressibility of ReB2 might suggest its potential application aspressure-proof conductors.