A nickel/gold (Ni/Au) pad structure of a semiconductor package and a fabrication method thereof are provided. The fabrication method includes preparing a core layer; forming a conductive trace layer on the core layer; patterning the conductive trace layer to form at least one pad of the conductive trace layer; applying a conductive layer; forming a photoresist layer to define a predetermined plating region on the pad, wherein the predetermined plating region is smaller in area than the pad; forming a Ni/Au layer on the predetermined plating region; removing the photoresist layer and etching away the conductive layer; and applying a solder mask layer and forming at least one opening in the solder mask layer to expose the pad, wherein the opening is larger in area than the Ni/Au layer. The Ni/Au pad structure fabricated by the above method can prevent a solder extrusion effect incurred in the conventional technology.