The precipitation hardening behavior of Al-5.1Mg-0.15Cu alloy with 3.0Zn (wt%) addition at various temperatures has been systematically investigated in this study. The Al-5.1Mg-0.15Cu-3.0Zn alloy possesses the higher strength compared with traditional Zn-free alloy, when the alloy is treated at 363 K for 24 h and subsequently aged at 413 K for 25 h. The precipitation sequence of the hardening phase T-Mg-32(AlZn)(49) is investigated by differential scanning calorimetry tests, transmission electron microscopy and high-resolution electron microscopy. The different effects of natural aging and pre-aging on the precipitation hardening behavior are detailed. The formation of relatively stable Guinier-Preston zones of T-Mg-32(AlZn)(49) phase during pre-aging rather than natural aging results in higher mechanical strength during subsequent artificial aging at relatively high temperatures. When the Al-5.1Mg-0.15Cu-3.0Zn alloy is aged at 453 K, the peak hardness is from synergetic effect of hardening of S-Al2CuMg phase and coarsening of T phase. However, during two-step artificial aging treatment, the peak hardness results from fine, spherical and homogeneously distributed T phase in the absence of the S phase.