  In this Comment, we show that the experimental data reported by Li {\it et
al.} (Phys. Rev. B {\bf 77}, 134501 (2008)) do not support the phonon specular
reflection at low temperatures for high-T_c cuprates, because the phonon mean
free path \ell calculated from their thermal conductivity data is much smaller
than the averaged sample width above \sim 100 mK for both Nd_{2}CuO_{4} and
YBa_{2}Cu_{3}O_{6.0} single crystals.
