Some proteins have the property of self-assembly, known to be an important mechanism in constructing supramolecular architectures for cellular functions. However, as yet, the ability of double-stranded (ds) DNA molecules to self-assemble has not been established. Here we report that dsDNA molecules also have a property of self-assembly in aqueous solutions containing physiological concentrations of Mg2+. We show that DNA molecules preferentially interact with molecules with an identical sequence and length even in a solution composed of heterogeneous DNA species. Curved DNA and DNA with an unusual conformation and property also exhibit this phenomenon, indicating that it is not specific to usual B-form DNA. Atomic force microscopy (AFM) directly reveals the assembled DNA molecules formed at concentrations of 10 nM but rarely at 1 nM. The self-assembly is concentration-dependent. We suggest that the attractive force causing DNA self-assembly may function in biological processes such as folding of repetitive DNA, recombination between homologous sequences, and synapsis in meiosis.