The high quality, high purity and large size Lanthanum hexaborides (LaB6) single crystals have been successfully grown by optical floating zone method. The optimum crystal growth parameters are listed as follow: sample rotation rate is 30 r/min and the growth rate is 810 mm/h. The largest thermionic emission current density of (100) crystal surface is 44.36 A/cm2 at 1873 K. The work function at absolute zero is calculated to be 1.99 eV by Richardson line method, and the average value of effective work functions at different temperatures are calculated to be 2.59 eV. The field emission characteristic of single crystal LaB6 field emitting single tip show that the maximum field emission current density is 4.9106 Acm-2 and the field enhancement factor is calculated to be 41500 cm-1, indicating excellent field emission performance. Thus, the single crystal is a promising cathode material for practical applications regarding to its excellent thermionic emission or field emission properties.