The lattice constants of Nd14Fe78−xMnxC8compounds decrease with the increase of Mn contextxand have a minimum at aboutx=14. The Curie temperatureTcdecreases linearly and falls off below room temperature beyondx=14. The strong reduction of the saturation magnetization andTcare attributed to the antiparallel alignment of the Mn and Fe atoms moments. The behavior of magnetization and magnetization reversal in the high‐Mn‐containing samples at low temperature can be interpreted by the narrow domain wall effect. The relationship of the intrinsic coercive forcei Hcon temperature agrees well with the exponential formula of the narrow Bloch wall.