In this study, we fabricated pentacene organic thin film trasistors(OTFTs) which used aluminum oxide as the gate insulator. Aluminum oxide for OTFTs was deposited on glass substrate with a different deposition rate by E-beam evaporation. In case of the deposition rate of $0.1;{AA}$, the fabricated aluminum oxide gate insulating OTFT showed a threshold voltage of -1.36 V, an on/off current ratio of $1.9{ imes}10^3$ and field effect mobility $0.023;cm^2/V_s$.