An hypereutectic aluminium alloy with a composition of 16-18wt%Si, 4-5wt%Cu, 0.45-0.65wt%Mg, 0.3wt%Nd and Al balance (A390-0.3wt%Nd alloy) was processed by extrusion and aging at 200ºC for 10 hours. The effect of the thermomechanical processing and heat treatment on microstructure and mechanical properties was investigated for A390-0.3wt%Nd alloy. It was shown that the eutectic structure disappeared after the processing and heat treatment. In the meantime, the morphology of Al(Si,Mg) solid solution was transformed from dendritic to equiaxed and the Si particles were refned to have fne sizes of 1-10µm and dispersed in the Al(Si,Mg) matrix. Some of the Si particles became globular in shape, which can reduce stress concentration. The tensile strength of the A390-0.3wt%Nd alloy increased by 14.8% (211MPa to 242MPa) after extrusion and aging, and the elongation to fracture increased signifcantly from 0.5% to 13.3%. Thus, the mechanical properties of extruded and heat-treated A390-0.3wt%Nd alloy were improved after the extrusion and aging. The fracture mode is transformed from brittle fracture of as-cast alloy to ductile fracture of extruded and heat-treated alloy.