Using pentacene as an active material, the organic thin film transistors were fabricated on Si3N4/p-Si substrates by using RF-magnetron sputtered amorphous aluminium as the gate electrode contact, and using highly doped Si as the gate electrode and substrate with plasma-enhanced chemical vapor deposited (PECVD) silicon nitride as gate dielectric. Pentacene thin films were deposited by thermal evaporation on dielectrics as the active layer, then RF-magnetron sputtered amorphous aluminium was used as the source and drain contacts. Measurement results show that field effect mobility and threshold voltage are 0.043 cm2/(V·s) and 12.6 V, respectively, and on-off current ratio is nearly 1×103.