The structural, electronic, vibrational and thermodynamic properties of Zri,Hf CoH3 (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) are investigated using first principles approach based on the virtual crystal approximation (VCA). The results indicate the series Zri,Hf CoH3 have the similar physical properties. When Hf concentration increases gradually, the lattice parameter reduces and the thermodynamic stability first decreases and then increases, respectively. The calculated results of charge distributions and electron localization function (ELF) suggest that the interactions of H Co and H Zri... Hff are primarily metallic with a small covalent component. The band structure and the corresponding density of states (DOS) around the Fermi level (Ef) indicate the metallicity enhances and the electrical conductivity is better with increasing Hf content. The phonon density of states imply that with the increase of Hf content, the covalent interactions between H(4c2) and Zr1-xHfx are weakened, while the covalent interactions between H(8f(1)) and Zr1-xHfx basically remained unchanged (H(4c(2)) and H(8f(1)) represent the hydrogen atoms occupying 4c(2) and 8f(1) site, respectively), which is consistent with the results of charge density. Finally, the thermodynamic properties are obtained and discussed on the base of the obtained vibrational properties. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.