The role of Cu in precipitation behavior of 6000 series aluminum alloys has been investigated by isothermal calorimetry and transmission electron microscopy. The newly developed analytical techniques have been used to evaluate the precipitation kinetics in alloys with or without an initial pre-aging history. It has been found that Cu addition results in increasing the rate of precipitation when artificial aging is applied immediately after solutionizing and quenching. However, Cu has no significant effect on the kinetics of precipitation in alloys with the pre-aging history, while it produces finer microstructures in the pre-aged and then artificially aged alloys. These observations have been explained by the effect of Cu on increasing the rate of precipitate nucleation in 6000 series alloys. It has also been suggested that the effect of Cu on nucleation arises from the role of Cu on the cluster formation during and/or immediately after quenching.