The effect of artificial aging on the microstructure and hardness of an ultrafine-grained (UFG) Al-4.8%Zn-1.2%Mg-0.14%Zr (wt%) alloy was studied. The UFG microstructure with an average grain size of about 260nm was obtained by severe plastic deformation applying four passes of equal-channel angular pressing (ECAP) at room temperature. Then, artificial aging was performed on the ECAP-processed samples at 120 degrees C and 170 degrees C for 2h. In the ECAP-processed sample Guinier-Preston (GP) zones, MgZn2 precipitates and a high dislocation density were observed. After aging at 120 degrees C, coarse MgZn2 precipitates were formed in the grain boundaries, leading to softening, while the dislocation density did not decrease. Annealing at 170 degrees C yielded a growth of the matrix grains to 530nm with a significant decrease in the dislocation density. In addition, GP zones disappeared and MgZn2 precipitates were formed in both the grain interiors and the boundaries. This overaging of the precipitate structure and the decrease in the dislocation density resulted in a lower hardness than after annealing at 120 degrees C. It was found that the hardness reduction due to the change of the precipitate structure at 170 degrees C was higher than that caused by the decrease in the dislocation density.