An organic field-effect transistor based on pentacene semiconductor with CuI/Al bilayer electrode is investigated. The CuI layer, directly contacting the organic semiconductor layer, serves as the hole-injection layer. The overcoated metal layer is responsible for the reduction in contact barrier. Compared with the device with a single metal (Al, Au) layer used as the source-drain electrode, the device with CuI/Al electrodes considerably improves the hole mobility and the on/off current ratio and greatly reduces the threshold voltage. Results of X-ray photoelectron and ultraviolet/visible absorption studies reveal that the reduction in the contact barrier can be attributed to an electron transfer from pentacene and Al to CuI.