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Aluminum matrix composites (AMCs) reinforced by 5 wt% B4C particles were fabricated by direct laser deposition (DLD) technology. The phase composition, microstructure, and microhardness of the B4C/aluminum matrix composites (B4C/AMCs) were analyzed by OM, XRD, SEM, TEM, EBSD, microhardness test and molecular dynamics simulation (MD) in detail. The results show that acicular microstructure of AlB12 and Al4C3 are formed in the B4C/AMCs fabricated by DLD. With the addition of B4C, the crystallographic orientation of AMCs changes from (001) texture to random orientation distribution, and the microhardness increases by about 65%. During the DLD process, boron atoms and carbon atoms are released by B4C decomposition instead of diffusing through the solid–liquid interface between Al and B4C, and they react with the liquid Al to produce AlB12 and Al4C3 in the molten pool. Moreover, B and C atoms distribute along a strip orientation in the molten pool, which may be retained after the cooling to form the acicular microstructures. This work demonstrates the possibility and good prospects of obtaining AMCs reinforced with B4C, AlB12 and Al4C3 phases by the DLD technique and provides new insights into the microstructural evolution of the composites. [ABSTRACT FROM AUTHOR]