Precipitation behaviour in an Al-Mg-Si alloy aged at 403 K to 483 K was studied with respect to thermal stability and morphology of the metastable precipitates, using high resolution transmission electron microscopy (HRTEM), Vickers microhardness tests and differential scanning calorimetry (DSC) measurements. The quantitative analysis of the DSC measurements revealed that the change in the first exothermic peak (the peak P) of the metastable phase is proportional to the increases in the Vickers hardness. The HRTEM observations showed the four types of the precipitates in morphology during the isothermal ageing and the change in the peak P was mainly caused by the formation of the precipitates with irregular contrast and the ones with the network interior angle between 65 degrees and 80 degrees.