Solutionising and ageing heat treatment for a newly developed die-cast Al-Mg-Zn-Si alloy has been carried out in order to investigate the microstructural evolution, the improvement in mechanical properties and the relevant strengthening mechanisms. Based on the results obtained from the treatment at different temperatures and periods of time, the heat treatment was optimised to be at 490 degrees C for 30 min for the solutionising, and at 180 degrees C for 60 min for the ageing by considering the improvement of both the strength and ductility. Mg-32(Al,Zn)(49) intermetallic in the as-cast microstructure was dissolved into the alpha-Al phase during solutionising and fine precipitates were formed during the subsequent ageing to enhance the mechanical properties of the alloy. After the heat treatment, the samples made by high pressure die-casting exhibited yield strength of 320 MPa, UTS of 420 MPa and elongation of 4.5%, which was a significant improvement in comparison with that under as-cast condition. The improvement of the mechanical properties achieved by the solution and ageing treatment is favourable for the applications where lightweight and high strength are essential for structural thin wall castings. (C) 2015 Elsevier B.V. All rights reserved.