In this study, the microstructural evolution and mechanical flow of underaged and peak-aged 6061 aluminum alloys processed by equal channel angular pressing (ECAP) were investigated. The inhomogeniety of plastic flow is appreciably influenced by the hardening behavior of underaged and peak-aged 6061 al alloys. The corner gap was developed more pronouncedly in the underaged alloy than in the peak-aged alloy, consistent with the prediction by finite element analysis. The difference of the die corner filling behavior of underaged and peak-aged Al alloys can be related to the hardening behavior, which is strongly influenced by the difference of precipitates. One interesting observation is that some beta' precipitates are slightly curved or fragmented after ECAP. Precipitates in aged Al alloys are gradually fragmented and are likely to be dissolved after several passes of ECAP.