Organic thin-film transistor (OTFT) based on pentacene semiconductor with an embedded copper oxide (CuO) thin layer is investigated. With the 3 nm-thick CuO layer embedded in the pentacene semiconductor, the drain current of the OTFT increases more than 3 times compared with that of pentacene organic field-effect transistor without CuO layer, and the absolute threshold voltage reduces from -21 V to -7.9 V. The hole mobility and current on/off ratio are much improved. It is interpreted by the mechanism based on the analysis of the interface charge transfer between pentacene layer and CuO layer. Results of X-ray photoelectron reveal electron transfer from pentacene to high work function CuO and the formation of charge transfer (CT) complexes based on electron transfer near the contact of CuO and pentacene. The CT complexes between pentacene layer and CuO layer could reduce the exponential density of state near the band edge of pentacene and the pentacene bulk hole trap density, and enhance the pentacene bulk hole carriers injection, which leads to the improvement of the field-effect mobility of OTFT with CuO layer. Electrons are transfered from the highest occupied molecular orbital of pentacene to the thin CuO layer which can generate holes in pentacene. The generated hole has the same effect as that with applying negative gate voltage which influences the threshold voltage. The drain current of the device increases and the threshold voltage shifts from -21 V to -7.9 V. Therefore, the thin CuO layer that is directly embedded in the organic semiconductor layer, serves as the hole-injection layer, which is responsible for reducing the contact barrier of OTFT with CuO layer.