In this paper, we report on calculations of the electronic structure and the magnetic coupling in Fe_1-xCo_xB and (Fe _1-xCo_x)₂B iron-cobalt borides. In Fe_1-xCo_xB, the atomic magnetic moments decrease versus x the Co concentration and a ferromagnetic-nonmagnetic phase transition occurs at x∼0.9. For this series, our calculations show the presence of a small magnetic polarization on Co. For both series of borides, core hyperfine fields on atoms were also calculated. The Fe and Co spin magnetic moments μ_spin and core hyperfine fields B_hfcore exhibit similar concentration dependencies so that the ratio B_hfcore/μ _spin reveals not sensitive to the Co concentration.