A method for fabricating a packaging substrate includes: providing a base having a release film with two opposite surfaces, two first auxiliary dielectric layers enclosing the release film, and two metal layers disposed on the two first auxiliary dielectric layers, therewith an effective area defined on the two metal layers; forming an inner wiring layer from the two metal layers; forming on each of the two first auxiliary dielectric layers and the inner wiring layers a built-up structure having first conductive pads so as for two initial substrates to be formed on the opposite surfaces of the release film; removing whatever is otherwise lying outside the effective area; removing the release film; and forming dielectric layer openings in the two first auxiliary dielectric layers so as for two substrate bodies to be formed from the initial substrates, wherein a portion of the inner wiring layers are exposed to thereby function as second conductive pads.