A handheld electronic device (10) mainly consists of a cover unit (12), a base unit (14) and a sliding mechanism (100). The base unit has a supporting surface (14d). The cover unit has a supporting surface (12b) which is arranged on the supporting surface of the base unit in a way that the cover unit is capable of sliding with respect to the base unit between closed and open positions. The sliding mechanism is arranged between the cover unit and the base unit in order to guide the relative sliding between the cover unit and the base unit. The sliding mechanism substantially consists of a pushing positioner (110) and a sliding slot (120), which are respectively arranged at the base unit and the cover unit. The pushing positioner consists of a sleeve (112), a ball (114) received in the sleeve, a resilient element (116) received in the sleeve and pushing the ball so that the ball has a portion engaging in the slot and a round plate (118) engaging in the sleeve. The position of the round plate is adjustable thereby adjusting the pushing force of the resilient force acting on the ball. The slot further has front and rear recessed concavities (124, 122). The ball engages in the rear concavity at the open position, and in the front concavity at the closed position.