A handheld electronic device including a first body, a second body and a transmission mechanism is provided. The second body is stacked with the first body. The second body has a recess at a side facing the first body. The transmission mechanism includes a first plate, a second plate and a rotating member. The first plate is fixed on the first body. The second plate is slidingly coupled to the first plate. An end of the rotating member is pivoted on the second body, and another end of the rotating member is pivoted on the second plate. When the first body slides with respect to the second body toward the recess, the rotating member rotates and drives the first body enters the recess.