The properties of boron carbide-lanthanum boride composite material prepared by hot pressed sintering method was tested, lanthanum boride as a sintering aid for boron carbide is investigated in this paper. The study shows: the hardness of boron carbide-lanthanum boride increases with the content of lanthanum boride. When the content of the lanthanum boride is 6wt%, the hardness reaches its supreme value of 31.83Gpa，its hardness is improved nearly 20.52% compared to monolithic boron carbide. The content of the lanthanum boride does not affect greatly on flexibility strength. However，it gives much effect on fracture toughness. The fracture toughness changes like the form of saw-toothed wave as the content of lanthanum boride increases in the test. When the content of the lanthanum boride is 6wt%, the fracture toughness reaches its supreme value of 5.14MPa.m1/2, which is improved nearly 39.67% compared to monolithic boron carbide materials. The fracture scanning electric microscope analysis of boron carbide-lanthanum boride composite material finds that, with the increasing the content of lanthanum boride, the interior station of monolithic boron carbide is changed. The crystallite arrangement is so compact that pores disappear gradually. The main fracture way of boron carbide-lanthanum boride composite material is intercrystalline rupture, while the transcrystalline rupture is minor, which is in accordance with fracture mechanism of ceramic material. It appears that this change of fracture mode by the addition of lanthanum boride gives rise to the improvement of the fracture toughness.