We fabricate pentacene-based organic field effect transistors (OFETs) with Cu as source and drain (S-D) electrodes.The fabricated devices stored for ten hours under ambient atmospheric conditions exhibit superior performance compared with the as-prepared devices.The field-effect mobility increases from 0.012 to 0.03 cm2 V-1 s-1,and the threshold voltage downshifts from -14 to -9 V.The on/off current ratios are close to the order of 104.The improved performance of the stored devices is attributed to the formation of thin Cu oxide at the Cu electrodes/organic interfaces.These results suggest a simple and available way to optimize device properties and to reduce fabrication cost for OFETs.