  We numerically investigate the electronic transport properties of graphene
nanoribbons and carbon nanotubes with inter-valley coupling, e.g., in \sqrt{3}N
\times \sqrt{3}N and 3N \times 3N superlattices. By taking the \sqrt{3} \times
\sqrt{3} graphene superlattice as an example, we show that tailoring the bulk
graphene superlattice results in rich structural configurations of nanoribbons
and nanotubes. After studying the electronic characteristics of the
corresponding armchair and zigzag nanoribbon geometries, we find that the
linear bands of carbon nanotubes can lead to the Klein tunnelling-like
phenomenon, i.e., electrons propagate along tubes without backscattering even
in the presence of a barrier. Due to the coupling between K and K' valleys of
pristine graphene by \sqrt{3} \times \sqrt{3} supercells,we propose a
valley-field-effect transistor based on the armchair carbon nanotube, where the
valley polarization of the current can be tuned by applying a gate voltage or
varying the length of the armchair carbon nanotubes.
