A release layer material of cyclic olefin copolymers (COC) applied in flexible electrical devices represented by Formula (I) or (II) is provided. The invention also provides a substrate structure including the release layer. The substrate structure includes a carrier, a release layer overlying the carrier with one or more blocks with a first area, wherein the release layer includes cyclic olefin copolymers (COC) represented by the disclosed Formula (I) or (II), and a flexible substrate overlying the release layer and the carrier with a second area, wherein the second area is larger than the first area and the flexible substrate has a greater adhesion force than that of the release layer to the carrier. The invention further provides a method for fabricating the substrate structure. In Formula (I) or (II), X is 30-70, X+Y is 100 and R is —H, —CH3 or —C2H5.