Oxaliplatin is a third-generation platinum derivative which is especially efficient for gastric and colorectal cancer treatment. Most studies revealed that the target of oxaliplatin is DNA molecule. However, the key structures formed by oxaliplatin and DNA still controversial so far. In this study, the interactions between oxaliplatin and DNA in vitro have been studied by UV spectra and atomic force microscopy (AFM) so as to reveal the structural basis of its anticancer activity. The structural characteristic after the incubation oxaliplatin at different concentrations with DNA for different time was investigated on freshly cleaved highly ordered pyrolytic graphite (HOPG) surface by atomic force microscopy (AFM). Based on the observed results, the interaction mechanism between oxaliplatin and DNA was proposed. High resolution AFM observation indicated that oxaliplatin can induce structural changes of pUC19 DNA from the extended chains to the intertangled structures with many nodes, and finally to a compact particle after 24 h incubation. The present results revealed that electrostatic and coordination interactions are important to condense plasmid DNA to a compact particle which may significantly influence the activity and cytotoxicity of oxaliplatin.