The field effect in pentacene thin-film transistors was studied using bottom-contact devices with channel lengths below 10 nm. To suppress spreading current in these devices, which have a small channel width-to-length (W-L) ratio, we employed a pair of guarding electrodes as close as 20 nm to the two sides of the channel. The responses of these nanometer scale transistors exhibit good gate modulation. Mobilities of 0.046 cm2/Vs and on/off ratios of 97 were achieved in sub-10-nm transistors. We find that the device response is strongly influenced by the nature of the metal-semiconductor contact.