The objective of this study is to provide a better knowledge on the influence of ageing conditions on the properties of a Friction Stir Welded 6056 aluminium alloy. A weld performed on T78 condition and then naturally aged is compared to a weld performed on T4 material followed by a post-weld T78 ageing treatment. Microstructural results are discussed with taking into account the thermal cycles encountered during welding. Results of tensile tests indicate that the properties of both welds are explained by a strong localisation of plastic strain in the softest microstructural zones.