She told People she feels her most beautiful after a workout.
"It's funny, it's a really quick transition from not a care and now all of a sudden, we've got to really be mindful of what we put inside our bodies," she said. "And how we sleep and take care of ourselves. You can get away with a lot in your 20s."
It's the second such cover for Aniston who also received the accolade in 2004. Last year's winner was Sandra Bullock.