We report on ambipolar organic field-effect transistors (OFET) based on plithalocyanince composite heterojunction. The heterojunction consisted of n-type hexadecafluorophthalocyaninatocopper (F16CuPc) as the first active layer connected to gate insulator and p-type copper phthalocyanine (CuPc) as the second active layer covered on the top of F16CuPc. Compared with conventional normally off devices, these heterojunction OFETs can be operated in normally on mode. In addition, typical ambipolar electrical characteristics are observed in air ambient from a single heterojunction device by optimizing the thickness of the active layers. The result is attributed to two comparable concentrations of carriers under various gate voltages. © 2005 Elsevier B.V. All rights reserved.