Effects of isothermal heat-treatment at the semisolid temperature region, extrusion rate, and mold temperature on microstructure and fluidity of an Al-4.5wt.%Cu alloy were investigated. Microstructural change during isothermal heat-treatment was similar to that of a Mg-9wt.%Al alloy, but different from that of an Al-7wt.%Si alloy. It was found that the solid phases tend to agglomerate with irregular shape, size and distribution, and the solid boundaries didn`t disappear, as was found in a Mg-9wt.%Al alloy. And nonuniform distribution of solid phase and segregation of solute atoms after extrusion were found. Fluidity was found to be much lower than those of Al-7wt.%Si and Mg-9wt.%Al alloys.