  We investigate the phonon thermal conductance of graphene regarding the
graphene sheet as the large-width limit of graphene strips in the ballistic
limit. We find that the thermal conductance depends weakly on the direction
angle $\theta$ of the thermal flux periodically with period $\pi/3$. It is
further shown that the nature of this directional dependence is the directional
dependence of group velocities of the phonon modes in the graphene, originating
from the $D_{6h}$ symmetry in the honeycomb structure. By breaking the $D_{6h}$
symmetry in graphene, we see more obvious anisotropic effect in the thermal
conductance as demonstrated by dimerite.
