We presented a fast response organic photodetector based on high mobility material rubrene and C60 as donor and acceptor, respectively. It was found the diffusing time of the excitons limited the bandwidth in photodetectors composing of rubrene/C60 bilayer heterojunction. As introducing a mixing layer of rubrene and C60 between rubrene/C60 heterojunction, the response speed is greatly improved. The presented organic photodetectors finally showed a bandwidth of 15.6MHz under a small reverse bias of −2V. Moreover, the specific detectivity exceeded 10 12 Jones and the dark current density was as low as 1.5nA cm −2 .