The fluidity evolution of an Al-10 vol.% B4C experimental composite during long holding periods has been investigated by using a vacuum fluidity test. It was found that the fluidity of the composite melt decreased with the increase of the holding time. The microstructure of the fluidity samples was examined by optical metallography, quantitative image analysis, and electron microscopy. Two secondary reaction-induced phases were identified and the volume fraction changes of the solid phases during the holding periods were quantified. The relationship between the fluidity, volume fraction, and surface area of solid phase particles was established. In addition, the particle distribution along the entire length was examined in the fluidity samples. The mechanism of the particle redistribution during flow and solidification is presently discussed.