Precipitates in A1-1.0mass%Mg2Si-0.5mass%Cu alloy aged at 423 K were observed by high-resolution transmission electron microscope (HRTEM) in order to investigate the effect of Cu addition on precipitation sequence of Al-Mg-Si alloys. There was a precipitate, which has different morphology from normal metastable phases in Al-Mg-Si alloys. It was a thin lath- or needle-shaped precipitate and not classified with a normal needle-shaped precipitate in Al-Mg-Si alloys.