The magnetic hardening process according to the heat treatment of a Fe-Cr-Co hard magnetic alloy was investigated. It could be described that the magnetic hardening process of this alloy was divided into three parts. In the heat treatment of the first stage, the thermomagnetic treatment, FeCo-rich (α₁) phases were precipitated. The aspect ratio of precipitates was determined in this stage. The heat treatment of the second stage caused precipitates to grow, improving the magnetic properties of the alloy. The alloy became brittle due to the rapid increase of the hardness by the second stage. In the heat treatment of the third stage, the concentration of the matrix (α₂) and the precipitates (α₁) changed gradually with little change in the shape and volume fraction of the precipitates. The coercive fores of the alloy increased largely in this last stage.