The hardness variations and precipitation behaviour during the three-stage ageing of three Al-Cu-Mg alloys were investigated using micro-hardness testing and differential scanning calorimetry (DSC). To facilitate the determination of an optimised heat treatment, a uniform design method was employed by which the influences of three-stage interrupted ageing on the hardness and precipitation can be elucidated with a limited number of experiments. It is found that optimised heat treatment with maximum hardness can be achieved for non-stretched materials by applying the method of three-stage ageing, but there is no obvious effect for stretched T351 materials. A long initial Stage I and a suitable Stage II ageing time are beneficial for hardness, and the Stage II ageing at 25 degrees C shows more beneficial effect than ageing at 65 degrees C. The hardness increase in Stage I and Stage III ageing is closely related to the S phase content.