DNA computing employs DNA molecule as a main resource to fulfill computing tasks. However, the concept of primary DNA computing unit keeps obscure. It is recently realized that there are multiple forms of basic DNA computing units, all with the fundamental property of nano-scale DNA fragments of Waston-Crick pairing. In this review five non-exclusive types of the main DNA computing units were summarized one by one, and they are Adleman unit, Tiling unit, Rothemund-Shapiro unit, Ribozyme unit and Paun unit. Adleman unit is actually just basic Waston-Crick pairing/ligation, or a simplified version of DNA assembly; Tiling unit is from various DNA tiles, with assembly as computing process; Rothemund-Shapiro unit is Turing machine-like autonomous DNA automaton. Ribozyme unit is an endeavor that may first lead to protein enzyme-free DNA automaton. There is another special DNA computing unit called here as “Paun unit”, which is the membrane computing unit. It is not easy to comment which unit is most promising in the future, but studies on all these units will certainly promote DNA manipulation technologies and thus development of novel nano-scale DNA computing devices.