Optical absorption spectra of BC3 nanotubes are studied within the gradient approximation. BC3 nanotubes exhibit rich absorption peaks in the overall frequency because of a lot of one-dimensional energy bands. The threshold absorption frequency is similar to 0.15y(0) for all BC3 nanotubes. Absorption peaks are mainly determined by the chirality, radius, and magnetic flux. BC3 nanotubes quite differ from carbon nanotubes in the low- and high-frequency absorption spectra. The calculated results are roughly consistent with the optical measurements, such as the threshold absorption frequency and the existence of the low-frequency absorption peaks. (c) 2006 American Vacuum Society.