We have fabricated organic transistors based on ink-jet printed copper as source and drain electrodes. Ink-jet printing of metal nano-particles is an attractive alternative to photolithography for patterning conductive metal lines, due to the low-cost, low-waste and simple process. For conductive copper electrodes at low temperature heat-treatment below 250°C, nano-sized copper particles of 50nm diameter were printed on the highly doped silicon wafer with 300 nm-thick silicon dioxide layer that acts as a dielectric layer. It was observed that the conductivity of the ink-jet printed lines was similar to that of bulk copper material even though the heat-treatment was performed at such a low temperature. To achieve smooth conductive path with high resolution, the printing conditions of the distance between printed droplets, stage moving velocity, and temperature substrates were optimized. Pentacene was evaporated on the printed copper electrodes to fabricate the organic transistor, since pentacene offers higher mobility, better on-off ratio, improved environmental stability, and better reliability than most other organic semiconductors. The measured carrier mobility and on/off ratio of the fabricated bottom-contact organic transistor show similar performance when compared with the transistors based on evaporated gold electrodes.