The DNA-binding domain of Myb consists of three imperfect tandem repeats, each of which contains three conserved tryptophans. The solution structure of the third repeat was established by NMR; it includes three helices maintained by a hydrophobic core formed by three tryptophans, together with two histidines, and the second and third helices form a helix-turn-helix-related motif. Here, we report the whole structure of the DNA-binding domain, as determined by NMR. Each of the first and second repeats resembles the third one in structure, retaining three helices. As a whole, the DNA-binding domain contains an entirely unique structure, a triple helix-turn-helix-related motif. To determine the role of each repeat in DNA-binding, a 16-mer DNA duplex that is specifically recognized by Myb was synthesized and then its complex with the DNA-binding domain has been investigated by 3D-NMR. The interaction mode between the whole DNA-binding domain and the duplex has shown that the third helix in each repeat likely locates in the majour groove of DNA. It is suggested that the Myb DNA-binding domain wraps around the DNA duplex by fitting the triply repeated helix-turn-helix-related motifs in the majour groove.