The La–Co co-substituted magnetoplumbite-type (M-type) ferrites A Fe 12 O 19 ( A = Ca, Sr and Ba, ion sizes Ca 2+ < Sr 2+ < Ba 2+ ) with Co compositions around 0.2 have been subjected to 59 Co-NMR. The results show that Co occupies the 4f 1 , 2a and 12k sites, and that the smaller the A ion, the more Co tends to occupy the 4f 1 minority spin site, which is effective in enhancing both uniaxial anisotropy and magnetisation. First-principles total energy calculations based on density functional theory (DFT) of undoped A Fe 12 O 19 and a supercell ( 2 × 2 × 1 of the unit cell) in which 1/96 of Fe 3+ is replaced by Co 2+ were performed to predict the stable structure and Co occupancy sites. The results show that regardless of A , Co is most stable when it occupies the 4f 1 site, followed by the 2a and 12k sites with energy differences on the order of 100 meV, and Co practically does not occupy the 2b and 4f 2 sites. As the A ion becomes smaller, the energy difference when Co occupies each Fe site tends to increase, and the Co occupancy of the 4f 1 site also increases. The site selectivity of Co can be roughly explained as a result of the difference in uniaxial strain along the c -axis associated with the difference in A . However, the influence of the A ion differs between the R and S blocks and the local strain also has a secondary effect on the Co distribution. Based on these results, the guidelines for improving the performance (anisotropy and magnetisation) of La–Co co-substituted M-type ferrite magnets with a limited amount of Co can be summarised as follows: It is effective to select as small A ions as possible and to post-anneal at low temperature or cool slowly to concentrate Co at the 4f 1 site in tetrahedral coordination.