The invention provides an interposer substrate and a method of fabricating the same. The method includes: etching a carrier to form a recessed groove thereon; filling a dielectric material in the recessed groove to form a first dielectric material layer, or forming a patterned first dielectric material layer on the carrier; forming a first wiring layer, a first conductive block and a second dielectric material layer on the carrier and the first dielectric material layer sequentially, with the first wiring layer and the first conductive block embedded in the second dielectric material layer; and forming a second wiring layer and a second conductive block on the second dielectric material layer. A coreless interposer substrate having fine pitches is thus fabricated.