A new testing method, dubbed Mold Immersed Rapid Solidification (MIRS), is proposed to investigate the distribution of fly ash particles in ADC6+5wt.% fly ash composites prepared by stir casting. In preparing the composite slurries, the fly ash was preheated at different temperatures, namely 400°C, 500°C, 600°C, 660°C, 700°C and 800°C, and then added into ADC6 melt at different flow rates, namely 0.10 g/s, 0.15 g/s, and 0.2 g/s. The stirring speed and melt temperature were respectively fixed at 300 rpm and 700°C. Optical micrographs reveal that MIRS effectively locks the position of fly ash, allowing investigations of particle dispersibility in the matrix. Further, both hardness and density tests were used to investigate the uniformity of the fly ash distributions in the composites. The experimental results show that higher preheating temperature and lower adding rate of fly ash diminish or avoid porosity induced by particle clustering in the matrix, leading better chemical reactions between fly ash and aluminum melt. The lowest porosity content of 1.04% and the highest Brinell hardness number were obtained in this study with fly ash preheated at 800°C and added at 0.10 g/s. This represents a 20.7% increase in BHN compared with raw ADC6