The lattice constants of Nd14Fe78-xMnxC8 compounds decrease with the increase of Mn context x and have a minimum at about x=14. The Curie temperature Tc decreases linearly and falls off below room temperature beyond x=14. The strong reduction of the saturation magnetization and Tc are 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 force i Hc on temperature agrees well with the exponential formula of the narrow Bloch wall.