Barium hexaferrites doped with Sm 3+ , Ba 1-x Sm x Fe 12 O 19 , x = 0.0, 0.1, were prepared by the conventional ceramic technology. The structure and electromagnetic properties of the calcined samples were studied using powder X-ray diffraction (XRD) and network analyzer (Agilent 8722ET). All the XRD patterns showed the single phase of the magneto-plumbite barium ferrite without other intermediate phase. The lattice parameters of ferrites doped with Sm 3+ decreased, indicating that the substitution of Sm 3+ occurred on Ba 2+ basis site and resulted in a contract of the crystal cell. The microwave electromagnetic properties of the samples were studied at the frequency range from 2 to 18 GHz. It was shown that ɛ' increased slightly, and the maximum of ɛ” appeared at higher frequency position with Sm 3+ doping. μ” and tanμ r were improved significantly, and the maxima were 2.3, 2.0, respectively. All the reasons were discussed using electromagnetic theory.