The Al-Mg-Si alloys for autobody sheet application are capable of providing a good paint bake hardenability. However, in practical usages, it is difficult to perform paint bake cycle immediately after solution heat treatment and a delay at room temperature before the paint bake cycle is usually unavoidable. Also, the condition of paint bake cycle is insufficient to exert completely their age hardenability, although the alloys have a hardening potential. Therefore, in order to improve the paint bake hardenability, it is necessary to prevent the detrimental effect of natural aging before paint bake cycle. In this study, effects of Cu and Sn alloying elements and pre-aging treatment on paint bake hardening in Al-Mg-Si alloys have been studied by means of micro-hardness measurement and transmission electron microscopy. The results showed that the paint bake hardenability of the Al-0.5Mg-1.3Si-0.1Zr alloy is enhanced by the addition of Cu and Sn alloying elements and the adoption of the preaging treatments. It suggested that the clustering process occurred during natural aging is effectively inhibited by Cu and Sn alloying elements and the preaging treatments.