We overgrew single-crystalline Zn(BO2)2 coatings on carbon nanotubes (CNTs) for the first time. Scanning electron microscopy and transmission electron mi- croscopy analyses revealed that the carbon nanotube-zinc borate composite rods are from tens to hundreds of nm in diameter. It is notable that the original curly tubes were straightened by the monocrystalline coatings. In addition, the crystal nucleation and growth on the surface of CNT were explained. We set a two-dimensional model, which is based on our experimental result, to qualitatively explain the mechanism of straightening the curly tubes by coating them with single-crystals. Scanning electron microscopy and transmission electron microscopy analysis revealed that the carbon nanotube-zinc borate composite rods are from tens to hundreds of nm in diameter. It is not able that the original curly tubes were straightened by the monocrystalline coatings. In addition, the crystal nucleation and growth on the surface of CNTs was explained. We set a two-dimensional model, which is based on our experimental result, to qualitatively explain the mechanism of straightening the curly tubes by coating them with single-crystals.