In this paper, a technique using mixed transition-metal oxides as contact interlayers to modulate both the electron-and hole-injections in ambipolar organic field-effect transistors (OFETs) is presented. The cesium carbonate (Cs2CO3) and vanadium pentoixide (V2O5) are found to greatly and independently improve the charge injection properties for electrons and holes in the ambipolar OFETs using organic semiconductor of diketopyrrolopyrrolethieno[3,2-b]thiophene copolymer (DPPT-TT) and contact electrodes of molybdenum (Mo). When Cs2CO3 and V2O5 are blended at various mixing ratios, they are observed to very finely and constantly regulate the Mo's work function from -4.2 eV to -4.8 eV, leading to high electron-and hole-mobilities as high as 2.6 and 2.98 cm(2) V-1 s(-1), respectively. The most remarkable finding is that the device characteristics and device performance can be gradually controlled by adjusting the composition of mixed-oxide interlayers, which is highly desired for such applications as complementary circuitry that requires well matched n-channel and p-channel device operations. Therefore, such simple interface engineering in conjunction with utilization of ambipolar semiconductors can truly enable the promising low-cost and soft organic electronics for extensive applications.