A mechanism of a semiconductor structure with composite barrier layer under redistribution layer is provided. A semiconductor structure includes a substrate comprising a top metal layer on the substrate; a passivation layer over the top metal layer having an opening therein exposing the top metal layer; a composite barrier layer over the passivation layer and the opening, the composite barrier layer includes a center layer, a bottom layer, and an upper layer, wherein the bottom layer and the upper layer sandwich the center layer; and a redistribution layer (RDL) over the composite barrier layer and electrically connecting the underlying top metal layer.