A method of fabricating an interposer substrate is provided, including: providing a carrier having a first wiring layer and a plurality of conductive pillars disposed on the first wiring layer; forming a first insulating layer on the carrier, with the conductive pillars being exposed from the first insulating layer; forming a second wiring layer on the first insulating layer and the conductive pillars; disposing a plurality of external connection pillars on the second wiring layer; forming a second insulating layer on the first insulating layer, with the external connection pillars being exposed from the second insulating layer; forming at least a trench on the second insulating layer; and removing the carrier. Through the formation of the interposer substrate, which does not have a core layer, on the carrier, a via process is omitted. Therefore, the method is simple, and the interposer substrate thus fabricated has a low cost. The present invention further provides the interposer substrate.