The effect of the addition of copper, silver and gold on age-hardening, total elongation and fracture morphology of Al–Mg–Si alloys was investigated by hardness measurement, tensile test, observation of fracture surface by scanning electron microscopy, and transmission electron microscopy. The addition of copper for the base and excess Si alloys increased the peak hardness of alloys, while the addition of silver or gold did not show any modification of peak hardness. The total elongation of alloys bearing copper, silver or gold was improved, when the tensile test was performed for peak aged alloys. Fracture surfaces of alloys bearing copper, silve or gold included the region of transgranular fracture, while those of alloys without additional elements were mainly intergranular fracture. Surface relief and fine slip bands were observed on fractured samples of alloys bearing additional elements. Mean width of precipitate free zones (PFZ) in alloys bearing additional elements was narrower than that without additional elements. According to results of this work, it is considered that additional elements of Cu, Ag and Au improve total elongation of peak-aged Al–Mg–Si alloy because prior deformation around grain boundaries during tensile deformation was controlled by the formation of narrower PFZ than alloys without these elements.