To improve the properties of high pressure die cast Al9Si3Cu alloy, melt treatment via high shear was employed to form fine in-situ MgAl2O4 particles which act as potential nuclei to refine α-Al grains whilst providing reinforcement to enhance the tensile strengths of the alloy. Experimental results showed that the naturally formed MgAl2O4particles in the alloy melt were approximately 1 μm in diameter and are very limited in number as observed in the concentrated samples obtained via filtration. With the implementation of the high shear technology on the alloy melt, it is possible to synthesize a large number of fine MgAl2O4 particles which were approximately 80 nm–300 nm in diameter. The yield strength of the alloy containing such in-situ sub-nano-sized particles was found to increase from 147 MPa to 169 MPa in the as-cast state. Following treatment with direct ageing at 155 °C only, the yield strength of the alloy treated with high melt shear was found to increase by 128 MPa, ultimately achieving a yield strength and ultimate tensile strength of 297 MPa and 423 MPa respectively. The improvement of the mechanical properties of the alloy is attributed to the fine and uniform microstructure achieved by the enhanced heterogeneous nucleation of both α-Al grains and α-AlFeMnSi intermetallic compound via the in-situ MgAl2O4 particles, and also to the enhanced precipitation strengthening trigged by the increased dislocation density caused by the in-situ MgAl2O4 particles