A package substrate and a method of fabricating the package substrate are provided. The package substrate may include an interposer having at least one conductive through via, a photo-sensitive dielectric layer formed on one side of the interposer, and at least one conductive via formed in the photo-sensitive dielectric layer and electrically connected to the conductive through via. By means of a photo lithography process with high alignment accuracy, at least one via with an extremely small diameter can be formed on the photo-sensitive dielectric layer and align with the conductive through via. Therefore, the conductive through via can have its diameter reduced as required, without considering the alignment with the at least one via. Accordingly, the interconnection density of the conductive through via on the interposer is increased.