  Authors of Phys. Rev. Lett. 103, 228702 (2009) claim that "The residual
degree gradient (RDG) method can enhance thesynchronizability of networks by
simply changing the direction of the links". In this paper, we argue that in
some case the RDG method will lead to the failure of synchronization
($R=\lambda^{r}_{2}/\lambda^{r}_{N}=0$). Additionally, we also propose a
so-called residual betweenness gradient (RBG) method to solve this problem.
