Iron-doped beta-rhombohedral boron was synthesized by 28 GHz microwave irradiation on a powder mixture of iron and beta-boron. beta-Boron strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to promote a reaction with iron dopant. The powder mixture was heated to 1800degreesC within 2 min by microwave irradiation, resulting in the formation of beta-rhombohedral boron interstitially doped with iron. The reaction proceeded rapidly without accompanying grain growth. The XRD analysis and the electrical conductivity measurements revealed successful incorporation of iron into two doping sites of beta-boron.