The fracture behavior of the Au/Cr line structures formed on a rigid-rodlike polyimide, biphenylenetetracarboxylic dianhydride-phenylenediamine (BPDA-PDA), film substrate has been investigated using a stretch deformation technique and compared with that of the Au/Cr line structures formed on a semiflexible polyimide, pyromellitic dianhydride-oxydianiline. In general, the effects of metal line dimensions on the deformation behavior can be attributed to the changes in the mechanical environment induced by metal confinement at the line interface. This gives rise to a fracture behavior and geometrical dependence of these two polyimide structures which are qualitatively similar in most aspects. However, the fracture energy and crack propagation rate of these two polyimide line structures are quantitatively different, with values about twice higher for the BPDA-PDA line structures. This cannot be accounted for solely by the different chemical bonding; instead, the high fracture toughness of the BPDA-PDA structure has to be attributed to its superior mechanical properties, particularly its plastic deformation characteristics, which are related to its molecular structure.