In this paper, the plasmid DNA was used as a target to evaluate the bioeffect of TiO2/Carbon Nanotube (CNT) composites. The conformational change and breakage of DNA induced by the composites were characterized by the integrated tools of electrochemistry, circular dichroism (CD), atomic force microscopy (AFM), and DNA electrophoresis. At the early stage of incubation, the DNA double helix conformation was substantially changed by TiO2/CNT composites. Both electrophoresis and electrochemistry results suggested the breakage and damage appeared on the native DNA molecules. When DNA was treated longer by TiO2/CNT, DNA molecules were broken into fragment. AFM images confirmed the process. The DNA damage was deemed to be a gradual process: supercoiled plasmid DNA was first damaged to nicked-circle structure, then further to linear form, and then DNA fragment. Copyright © 2011 John Wiley & Sons, Ltd.