A surface-mountable over-current protection device comprises one PTC material layer, first and second connecting conductors, first and second electrodes and an insulating layer. The PTC material layer has a resistivity less than 0.2 Ω-cm, and comprises crystalline polymer and conductive filler dispersed therein. The first and second connecting conductors are capable of effectively dissipating heat generated from the PTC material layer. The first and second electrodes are electrically connected to first and second surfaces of the PTC material layer through the first and second connecting conductors, respectively. The dissipation factor depending on the ratio of the total area of the electrodes and the conductors to the area of the PTC material layer is greater than 0.6. At 25° C., the value of the hold current of the device divided by the product of the area of the PTC material layer and the number of the PTC material layer is greater than 1A/mm2.