Effect of aging on activity of twinning in magnesium alloy AZ91 was investigated. The experimental results showed that stress-strain curves during reversed loadings were different between tension after compression and compression after compression. The difference in stress-strain behavior decreased with the increase in aging time. Crystal plasticity finite element analysis revealed that the difference of activities of twinning and detwinning resulted in the difference in stress-strain behavior during reversed loadings, which suggested that the decrease of difference in the stress-strain curves with the increase in aging time related to the decrease in the activity of twinning.