We demonstrate bipolar switching of organic resistive memory devices consisting of Ag/polymer/heavily-doped p-type poly Si junctions in an 8 x 8 cross-bar array structure. The bistable switching mechanism appears to be related to the formation and rupture of highly conductive paths, as shown by a direct observation of Ag metallic bridges using transmission electron microscopy and energy-dispersive X-ray spectroscopy. Current images of high-and low-conducting states acquired by conducting atomic force microscopy also support this filamentary switching mechanism. The filamentary formation can be described by an electrochemical redox reaction model of Ag. Our results may also be applied to other kinds of organic materials presenting similar switching properties, contributing to the optimization of device scaling or memory performance improvement.