Marked effect of imposed low frequency oscillations with amplitudes of 6.6 × 10−4−11.6 × 10−4 during creep of pre-aged Al-40 wt.% Zn alloy samples under constant stress at 423 K, was observed. Increasing the frequency or amplitude at a certain temperature, increased both n and ${\dot \varepsilon}_{\rm st}$ but decreased β values. The value of the ratio γ (0.75−0.77) shows the high dependence of the steady creep on the transient creep stage. The results were explained in view of the nature of the thermally induced structures during aging and their mode of interaction with lattice defects generated by the applied stress and the imposed oscillations. Grain boundary sliding and grain boundary diffusion of Zn and Al are suggested as the rate controlling mechanisms for both the transient and steady creep stages, respectively.