  Zhang et al. reported in [Phys. Rev. B 77, 241402(R) (2008)] a theoretical
study of the optical spectra of monolayer graphene employing the Kubo formula
within a tight-binding model. Their calculations predicted that at high
frequencies the optical conductivity of graphene becomes strongly anisotropic.
In particular, at frequencies comparable to the energy separation of the upper
and lower bands at the $\Gamma$-point, the optical conductivity is strongly
suppressed if the field polarization is along the zigzag direction while it is
significantly high for the armchair one. We find that, unfortunately, this
result is just a consequence of the incorrect determination of the current
operator in k-space. Here, we present the standard scheme to obtain this
operator correctly. As a result, we show that the optical conductivity of
monolayer graphene is indeed isotropic, which is consistent with the results of
other (both theoretical and experimental) studies in the literature.
