In this paper

conductive atomic force microscope(c-AFM) is employed to investigate the conductance of composite structure of reduced graphene oxide(rGO) and double strand DNA molecule on isolated mica substrate

in torsional resonance tunneling current mode.The results indicate that graphene trends to absorb double strand DNA molecules to form composite body

and the c-AFM detections indicate that such a composite body is ineffective in long distance electron transfer

due to the high electric resistance of DNA.This work is of significance for explorations of applying graphene and DNA into molecular devices

and our measurement method is of practical value.