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The mechanical properties and microsctructures of the 6156 Al alloy subjected to a thermomechanical treatment consisting of under-ageing at 180 ℃ for 1 h, cold-rolling at room temperature with the thickness reductions of 50~80% and re-ageing at 100 ℃ for 48 h were investigated by optical microscopy, tensile test, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The tensile test indicates that the alloy can achieve high strength and relatively good ductility simultaneously and the ultimate strength, yield strength and elongation to failure are 515~564 MPa, 472~551 MPa and 5.7~11.1%, respectively. The microsctrucural observations show that the high strength results from the combination of deformation and precipitation strengthening and increases with increasing the thickness reductions. The improved ductility is attributed to both the decreased dislocation density and increased precipitation strengthening. The fractographies of the alloy consist of intergranular and dimple-induced transgranular fracture and the dimples have shallow depths and increased number with increasing the thickness reductions.