Abstract The magnetic properties of SmFe 7 − x B x compounds for 0 ≤ x ≤ 0.5 have been investigated. The main phase has the rhombohedral Th 2Zn 17 type structure for 0 ≤ x ≤ 0.2 and the tetragonal Nd 2Fe 14B structure for x = 0.5. X-ray diffraction analyses of aligned powders show that the easy direction of magnetization is perpendicular to the c-axis. The saturation magnetization at 4.2 K increases with increasing B content for 0 ≤ x ≤ 0.1. 57Fe Mössbauer spectra of SmFe 7 − x B x for 0 ≤ x ≤ 0.5 have been measured at 77 K and room temperature. The 57Fe hyperfine field increased with increasing B content for 0 ≤ x ≤ 0.1 at room temperature. By B substitution, the Fe moment increases for 0 ≤ x ≤ 0.1. The average Fe moment obtained from the 57Fe hyperfine field increases strongly from 1.59 μ B for SmFe 6.9B 0.1 to 2.01 μ B for SmFe 6.5B 0.5 at room temperature due to increasing B content and change of the crystal structure.