In order to improve mechanical properties of age-hardenable Al alloys markedly based on the conventional processing conditions, a novel thermo-mechanical treatment, which was consisted of pre-ageing, cold-rolling and re-ageing, was proposed in the present work. The effects of the thermo-mechanical treatment on microstructure evolution and mechanical properties of the 6061 Al alloy were investigated further by hardness measurement, tensile testing, XRD, TEM and HRTEM. It was shown that both the increased strength and elongation of the 6061 Al alloy were achieved simultaneously by the thermo-mechanical treatment. Especially, after the combination of under-ageing at 180 ℃ for 2 h, cold-rolling with the thickness reduction of 75% and re-ageing at 100 ℃ for 48 h, the ultimate tensile strength and yield strength of the alloy were 560 and 542 MPa, respectively, and the elongation was about 8.5%. The strength increment of the thermo-mechanically treated 6061 Al alloy was attributed mainly to the additional introduction of dislocation strengthening, dislocation cell strengthening and high Taylor factor value except for precipitation strengthening as compared to the conventional peak-aged alloy. Furthermore, the improved elongation of the thermo-mechanically treated 6061 Al alloy was mainly due to both the re-precipitation of strengthening precipitates and slight recovery of dislocations during re-ageing, which increased the accumulation ability of dislocations during the tensile deformation.