Thin electrolyte films of yttria-doped ceria (YDC, Ce0.8Y0.2O1.9) were fabricated on green substrates of NiO-YDC by a screen-printing technique. Dense YDC thin layers were subsequently formed on the porous NiO-YDC substrates after the green bi-layers were co-sintered at 1350 degreesC. The YDC films were about 15 mum as characterized by scanning electron microscope. With Sm0.5Sr0.5CoO2.75 as cathodes, single cells were tested from 450 to 650 degreesC with humidified (3% H2O) hydrogen as fuel and air as oxidant. Open circuit voltage of 0.97 V was obtained at 450 degreesC, indicating negligible gas permeation through the YDC thin films. Maximum power density of about 360 mW/cm(2) at 650 degreesC was achieved with current density of about 800 mA/cm(2). Electrochemical characterizations of the single cells indicate that it is critical to reduce the electrode polarization by developing new electrode materials as well as fabrication processes for solid oxide fuel cells that are operated at intermediate temperature.