An IC package mainly includes a substrate having slot(s), a chip, a protective encapsulant, a stiffening encapsulant, and a plurality of external terminals. The Young's modulus of the stiffening encapsulant is greater than the one of the protective encapsulant and the curing shrinkage of the stiffening encapsulant is smaller than the one of the protective encapsulant. The protective encapsulant is formed on one of the surfaces of the substrate for encapsulating the chip. The stiffening encapsulant protrudes from the other surface of the substrate where the external terminals are disposed. Moreover, the stiffening encapsulant is formed inside the slot and is contacted with the chip. Since the stiffening encapsulant is embedded and formed inside the slot, therefore, the contact area of the stiffening encapsulant with the substrate is increased to enhance the warpage resistance of the IC package.