The characteristics of organic thin-films transistors (OTFTs) with the anodized aluminum oxide insulator have been investigated. The OTFT with the barrier-type aluminum oxide insulator exhibited the mobility of 0.09 cm(2)/Vs, the subthreshold slope of 1.3V/decade, the threshold voltage of -2.2 V, and the on/off ratio of 7.7 x 10(4), which are superior to those for the device with the porous-type insulator. The smooth surface of the barrier-type film is found to contribute to a long range hopping of charge carriers in the conducting channel by decreasing the activation energy for the conduction of charge carriers.