Aluminum matrix Boron carbide (Al/B4C) composites is an important thermal neutron shielding material. In order to prepare B4C/6061Al composite with higher density, Al/B4C composites reinforced by 30wt.% B4C particle were fabricated via hot isostatic pressing in the semi-solid temperature range of 6061Al. Microstructures, mechanical properties and densification mechanism of the composites were investigated. The results indicate that the density of Al/B4C composite is very close to the theoretical density with the usage of semisolid hot isostatic pressing. Although the tensile strength can reach up to 300 MPa, the ductility of the semisolid HIP Al/B4C composite is not good mainly because of the high content of 30wt%B4C. Aluminum liquid phase is observed indirectly which mainly formed during semisolid temperature range of 6061Al, it is helpful to improve the combination between the B4C particles and the 6061Al matrix. The increasing of density and Strength property mainly also due to the effect of aluminum liquid flows into the internal gaps under the hot isostatic pressing condition with high temperature and high pressure.