We have measured the temperature-dependent thermal conductivity kappa(T) of boron-carbon-nitride (B-C-N) and boron nitride (BN) nanotube mats. The thermal conductivity of B-C-N nanotubes is phonon dominated and reflects dimensional effect below 70 K. We employ a new analysis method to estimate the intrinsic kappa(T) of BN nanotubes converted from B-C-N nanotubes, and find that at room temperature kappa(T) of a multiwalled BN nanotube is within 0.04-0.32 times that of a multiwalled carbon nanotube. The lower kappa(T) of BN nanotubes than that of carbon nanotubes may be due to the enhanced isotope disorder effect in one dimension. (c) 2005 American Vacuum Society.