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The method of calciothermic reduction of B4C was proposed for preparing CaB6. The phase transition and morphology evolution dur-ing the reaction were investigated in detail. The experimental results reveal that Ca first reacts with B4C to generate CaB2C2 and CaB6 at a low temperature and that the CaB2C2 subsequently reacts with Ca to produce CaB6 and CaC2 at a high temperature. After the products were leached to remove the byproduct CaC2, pure CaB6 was obtained. The grain size of the prepared CaB6 was 2–3 μm, whereas its particle size was 4–13 μm; it inherited the particle size of B4C. The residual C content of the product was decreased to 1.03wt％ after the first reaction at 1173 K for 4 h and the second reaction at 1623 K for 4 h.