The boron-rich rare-earth boride NdB 6 , which is a pure phase without the intergrowth of other phases, has been obtained for the first time by the reduction of rare-earth oxide with elemental boron under the extreme conditions of high pressure and high temperature. The synthesis can be carried out in air from the beginning to the end without the protection of inert gas, and the synthesis can be completed rapidly. The residual rare-earth oxide and the elemental rare-earth metal produced in the process of synthesis can be easily removed by leaching them in HC1.