Disclosed is a method of fabricating a package substrate using electroless nickel plating. The method includes preparing a base substrate, on which an internal layer circuit pattern is formed through a predetermined masking process; forming an insulating layer on the base substrate to achieve interlayer electric insulation; forming a first via hole through the insulating layer to achieve interlayer electric connection; forming a seed layer on the insulating layer through which the first via hole is formed; and forming an external layer circuit pattern on the seed layer through the other predetermined masking process. The seed layer is partially or selectively flash etched so as to prevent via open and undercut formed at the external layer circuit pattern.