Abstract: The 4. 4 %TiC_P/7075 matrix composite was prepared by in-situ reactive near-liquidus casting. The sectioned specimens were heat-treated isothermally at 600℃ between the two-phase (liquid-solid) region (477～ 635℃) of the near-liquidus cast 7075 alloy for 10, 20, 40 and 60 min, then quenched in water. The grain size was measured using the software Image Pro Plus and the effect of in-situ TiC_P on the reheating structure was analyzed. The result shows that the in-situ TiC_P with fine and globular can not only keep near-liquidus cast 7075 alloy microstructure, but also cont rol evidently grains growth by pinning effect during reheating. Dealing under the same condition , the grain sizes of TiC_P/7075 matrix composites are 30～40 μm less than those of 7075 alloy, which is very suitable for thixoforming.