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LaB_<6.0-6.14> and CeB_<6.0-6.12> crystals were prepared by the floating zone method. Their thermionic emission properties were examined in the temperature range of 1100-1500℃. Increasing the boron contents hardly influenced the emission characteristics, but decreased the evaporation rate which determines the life time of the cathodes. Therefore, the boron-rich LaB_6 and CeB_6 crystal cathodes are expected to have longer life time than the stoichiometric commercial cathodes.