A handheld electronic device, including a first body, a second body, a sliding module and at least one rotating module. The sliding module is coupled to the second body, so that the second body is suitable for sliding between a first position and a second position. The rotating module is coupled between the first body and the sliding module, so that the second body is suitable for rotating relative to the first body. The second body receives a force to rotate from the second position to a critical position. When the second body passes the critical position, the rotating module releases a potential stored when the second body receives the force to drive the second body to rotate to a third position and form an included angle between the first body and the second body. A sliding mechanism assembly is also disclosed.