Under normal conditions in order to improve the mechanical properties, the solution and an artificial ageing treatment are usually adopted in Al-Si-Mg and other Al-Cu alloys. This work presents a new technology, the as-cast ageing process for Al-Si-Mg and Al-Cu alloys. As-cast ageing process means such an artificial ageing treatment process without solution treatment. As-cast ageing process for Al-Si-Mg alloy is such a process in which the ageing temperature is at 155 C and keeping the temperature for 5 hours before cooled to room temperature. The as-cast ageing process for an Al-Cu alloy is that the ageing temperature is 295 C and keeping the temperature for 4 hours before cooled to room temperature. The mechanical properties of an AI-Si-Mg alloy after as-cast ageing treatment are close to that of 356 alloy treated by a solution and ageing treatment process. The mechanical properties of Al-Cu have been improved greatly by the as-cast ageing process. Studies were carried out on the effect of Be, MM and Sri on the critical temperature for the as-cast ageing process. The effects of different factors on the enthalpy changes of Al alloys and on the phase transformation kinetics during the low temperature ageing have been studied. The phase transformation process of an Al-Si-Mg alloy during the low temperature ageing process has been investigated by differential scanning calorimetry and scanning electron microscopy (SEM). The strengthening mechanism for those alloys at low temperature aging condition has been studies in the work.