In order to explore the DNA damage mechanism induced by endogenous factors and the activities of the antioxidants, an electrochemical biosensor was fabricated based on multi-walled carbon nanotubes (MWCNTs) and hemoglobin (Hb). When the glassy carbon electrode (GCE) modified with chitosan (CS), double stranded DNA (dsDNA), MWCNTs and Hb was subjected to a negative potential of -1.4 V in PBS of pH 5.8, the dissolved oxygen was reduced to hydrogen peroxide (H2O2) on the surface of the MWCNTs. Then the H2O2 reacted with Hb and formed a Compound I (Hb-C-HOOH) and H2O. The Compound I, a highly oxidizing specie, can be used as reactive oxygen species (ROS) and to damage DNA in situ in the membrane. The ascorbic acid (AA) can protect the DNA from damage by scavenging the H2O2 to inhibit the formation of the Compound I. The ds-DNA oxidative damage degree and the protective effect of AA from DNA damage were monitored by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and ultraviolet-visible spectroscopy (UV-vis). The consistent conclusions were obtained by electrochemical methods and UV-vis. The studies may be helpful for understanding the DNA damage mechanism, the protection of AA from DNA damage in vivo, and the toxicity of nanomaterials in organism. (C) 2016 Elsevier B.V. All rights reserved.