. To investigate the effects of thermomechanical treatments (TMT) on the microstructure and properties of Al-Mg-Si alloy, the cold deformation on the ageing precipitation of a solution-treated Al-Mg-Si alloy was studied. The results shows that the time of reaching the peak hardness is shortened with the increasing deformation and an obvious increasing in the peak hardness and tensile strength are occured with higher amount of deformations. The microstructures of peak hardness reveals that the average size of the precipitates becomes smaller in size and greater in number in the alloy with 50% deformations. The study is also carried out to investigate the changes in resistivity of alloys during the ageing time in the alloy with and without 50% deformation. It was found that there are rapidly increasing in resistivity followed by decreasing with the onset of ageing time in both cases. However, the extent of increasing and decreasing in resistivity is much stronger in the alloy with 50% deformation. These results were discussed according to the effects of cold deformation on the dislocations and precipitates in the Al-Mg-Si alloy.