An over-current protection device includes two metal foils and a PTC material layer laminated therebetween. The PTC material layer has a volume resistivity between 0.07 Ω-cm and 0.32 Ω-cm. The PTC material layer includes a crystalline polymer, a conductive ceramic carbide filler of a particle size between 0.1 μm and 50 μm and a volume resistivity less than 0.1 Ω-cm, and a carbon black filler. The weight ratio of the carbon black filler to the conductive ceramic carbide filler is between 1:90 and 1:4. The conductive ceramic carbide filler and the carbon black filler are dispersed in the crystalline polymer. The resistance ratio R100/Ri is between 3 and 20.