Aluminum boron carbide particulate reinforced composites are advanced materials which have superior mechanical properties, and especially have the capability to capture neutrons. The liquid mixing process is one of the methods to produce economically and effectively the metal matrix composites. However, it was found that the fluidity of the composites was instable during liquid holding and casting. To examine the fluidity evolution over the time, the melt of an Al-10%B4C composite was hold at a constant temperature for a long period, and the fluidity was evaluated by means of a vacuum fluidity test. The microstructure of the fluidity test samples was examined by optical and electron microscopes. It is found that the interfacial reaction products between B4C and Al-matrix play an important role for the deterioration of the composite fluidity.