  We report a study on the thermal conductivity (\kappa) of Tb_2Ti_2O_7 single
crystals at low temperatures. It is found that in zero field this material has
an extremely low phonon thermal conductivity in a broad temperature range. The
mean free path of phonons is even smaller than that of amorphous materials and
is 3--4 orders of magnitude smaller than the sample size at 0.3 K. The strong
spin fluctuation of the spin-liquid state is discussed to be the reason of the
strong phonon scattering. The magnetic-field dependence of \kappa and
comparison with Y_2Ti_2O_7 and TbYTi_2O_7 confirm the magnetic origin of this
phonon-glass-like behavior.
