Using first-principles phonon calculations and the Quasi-Harmonic approach, the structural, energetic and dynamical properties of ordered (DO3) and disordered Fe25at.%Ni bcc alloys have been investigated. All calculations are based on the Projector Augmented Wave (PAW) method of the Density Functional Theory (DFT). The Special Quasirandom Structure (SQS) method has been used to model the disordered structure. It is found that 25at.% Nickel substitution increases the lattice parameter and consequently decreases the C-11 and C-12 elastic constants and bulk modulus of bcc-iron. Simultaneously, the C-44 elastic constant increases with Ni substitution. The calculated formation energies of the disordered structures at O K are lower than those of the ordered one. The predicted phonon spectra reveal the dynamical stability of both SQS and DO3 structures. Finally, the Gibbs free energy functions indicate that the relative stability of the disordered structure vs the ordered one increases with increasing temperature. (C) 2016 Elsevier B.V. All rights reserved.