  I estimate maximum thermal conductivity $\kappa$ of a perfectly aligned
bundle of single wall carbon nanotubes. Each row of aligned nanotubes has a
discrete structure. It consists of segments of nanotubes with length $L$. The
spacing between the segments block the phonon path through the row. Only the
scattering due to the finite length of the segments is taken into account. The
result is that the 'effective'' mean free path is of the order of $L/7$. For 1
micron tubes (10,10) we get maximum value of $\kappa\approx 300$W/m K at room
temperature. This result is in a reasonable agreement with the experiment by
Hone {\it et al.} assuming that in their samples $L\approx 1\mu{\rm m}$
