TEM in-situ straining tests on micro specimens of 6056 T6 alloy have been performed to determine the micro-mechanisms in relation with the mechanical strength. The dislocations are anchored along pinning lines and move by jump between these lines. The physical parameters, which describe these pinning lines have been determined. Evidence is found for activation of cross-slip even at room temperature. The cross-slip has been mainly observed between octaedric planes but also between octaedric and cubic planes. The relationship between these mechanisms and the mechanical properties is discussed. © 2013 EDP Sciences.