[en] We study the effects of longitudinal acoustic and optical phonon scatterings on the transport properties of carbon-nanotube devices with micron-order channel length, within the Kubo formula. We investigate the effects of the phonon scatterings with various wave numbers on the conductance. Furthermore, we evaluate the device properties of carbon nanotubes using the cut-off frequencies when the acoustic phonon is excited in the devices. The results presented here indicate the implication to high performance nanotube transistors