The different volume fractions of TiC particle reinforced 7075 aluminum matrix composites under the conditions of semi-solid state have been fabricated by the in-situ reactive liquidus method. The as-cast microstructure of the composites have been observed by OM （optical microscope） and TEM （transmission electron microscope）

and effects of TiC particle on the microstructure of the composites were investigated by measuring the average grain size with a mean linear intercept method. The result showed that TiC particle can obviously refine and spheroidize the as-cast microstructure of semi-solid Al matrix composites. The equaixed structure with average grain size of 35.6 μm in the composites can be observed with 2.5% TiC particle.