We have investigated the magnetic properties of liquid‐quenched Pr15(Fe1−xCox)77B8over the whole range of Co concentrations. The coercivityHciand the remanenceJrdepend on the Co content in the same way as in Nd15(Fe1−xCox)77B8. For the ternary Pr‐Co‐B a highHci, as large as 15.2 kA/cm, has been obtained. In Co‐rich alloys the temperature dependence ofHciandJris limited by the spin reorientation at temperatures well below the Curie point. Except for Pr15Co77B8, annealing of overquenched ribbons leads to substantially lower coercivities. The possible microstructural origin of the magnetic hardening is discussed.