Abstract: The semi-solid Al₂O_3P/Al-Cualloys cast with in-situ near-liquidus casting technology were reheated, and its microstructural evolution and grain growth mechanism were studied by means of optical microscope. The grain sizes and the volume fraction of liquid were measured using Image Pro Plus , and compared with theoretical calculation. The result s indicate that under the combination of reheating temperature 590℃and holding time 10～60 min, with no Al₂O₃ particles the Al-6. 8 %Cu mat rix alloy has average grain size of 89～132 μm and liquid volume f raction 14 %～26. 8 %, while 3. 6 wt%Al₂O_3P/Al-6. 8 %Cu composite has average grain size of 73～107 μm and liquid volume fraction of 11. 6 %～ 20. 9 %. The in-situ Al₂O₃ particle can control the grain sizes and the volume fraction of liquid growth evidently during reheating. Thus, the conclusion will offer one kind of new mentality for optimizing semi-solid microstructure.