The characteristic of an organic field effect transistor (OFET) depends strongly on both properties of an organic film and a carrier injection phenomenon. We have investigated the influence of carrier injection at the interface between the source electrode and the organic film on the characteristics of an OFET using a rubrene single crystal. The mobility estimated from the transfer characteristic of the OFET depended strongly on the channel length and the thickness of the rubrene single crystal although the mobility is intrinsically independent of the dimensions of an OFET. On the other hand, the temperature dependence of the saturation drain current was in good agreement with the thermal-field emission theory. These suggest that OFETs are controlled by not only the carrier accumulation at the channel but also the carrier injection.