A process for fabricating a circuit substrate is provided. A patterned conductive layer having an inner pad is provided on a base layer, a dielectric layer is disposed on the base layer and covers the patterned conductive layer, and a covering layer is disposed on the dielectric layer. A part of the covering layer is removed by dry etching to form a first opening. A part of the dielectric layer exposed by the first opening is removed to form a dielectric opening exposing a part of the inner pad. A patterned mask having a second opening to expose a part of the inner pad is formed on the covering layer. A conductive structure including a conductive block filling the dielectric opening, an outer pad filling the first opening and a surplus layer filling the second opening is formed. Finally, the patterned mask, surplus layer and covering layer are removed.