Abstract: The room and high temperature mechanical strength of the hot-pressed B4C ceramic are studied. The results show that the hot-pressed B4C ceramic sintered at 2 150℃for 10 minutes from the jet-milled powders(3. 85μm), has the density of 91. 6%T. D. (T. D.--theoretic density), and the room temperature bending strength andcompression strength are 280 and 1 463 MPa, respectively. However, the hot-pressed B4C ceramic from the vibratory-milled powders(0. 35 μm) has a relative density of 99. 7% T. D., and the room temperature bending strength and compression strength are 515 and 2 870 MPa, respectively. The bending strengths of both ceramics decrease slowly with increasing temperatures, but the compression strengths decrease quickly below 600℃ and decrease slowly above 600℃. At last, the relationship between mechanical strength of both B4C ceramics and temperature and porosity is discussed by use of the empirical formulae.