A top contact n-type organic field-effect transistor with low operating voltage was fabricated by employing Ta2O5/PMMA as the double insulators and PTCDI-C12 as the semiconductor active layer. The Ta2O5 layer was prepared by using simple economical anodization technique and the PMMA layer was prepared by using the spin-coating method. Compared with the OFET with single Ta2O5 insulator,the device with double insulators shows obviously better electrical performance. It has a field effect electron mobility of 0.063 cm2/Vs,an on/off ratio of 1.7×104 and a threshold voltage of 2.3 V.