An over-current protection device comprises two metal foils and a PTC material layer laminated between the two metal foils. The PTC material layer essentially comprises a polymer matrix and a conductive filler. The polymer matrix at least comprises a first crystalline polymer, e.g., LDPE, and a second crystalline polymer, e.g., PVDF, in which the melting temperature of the second crystalline polymer subtracting the melting temperature of the first crystalline polymer is equal to or more than 50.degree. C. The conductive filler is selected from metallic grain of a volumetric resistivity less than 500 .mu..OMEGA.-cm, and is distributed in the polymer matrix. The initial volumetric resistivity of the PTC material layer is less than 0.1.OMEGA.-cm, and the trip temperature of the PTC material layer at which the resistance thereof increases to 1000 times the initial resistance subtracting the melting temperature of the first crystalline polymer is less than 15.degree. C.