Primary creep tests were performed on commercially pure aluminum in the temperature range of 0.32 - 0.38 T_m. The activation energies were obtained by usual method using different specimens (Q_c) and the temperature cycling method (Q_(T.C.)). The relation of activation energy with effective stress, σ_e, has been analysed. The activation length, ℓ_c^*, obtained from Q_c is linearly proportional to the inverse of affective stress. But no such relationship is observed in ℓ_(T.C).^* obtained from Q_(T.C.). The Q_c seems to have better physical meaning compared to Q_(T.C).