The electric field effect on the transport properties of ultrathinY0.9Pr0.1Ba2Cu3Oychannels was examined as a function of channel thickness and channel length. Samples having a planarYBa2Cu3OxultrathinY0.9Pr0.1Ba2Cu3Oy-YBa2Cu3Oxstructure with a gate electrode on the backside of theirSrTiO3substrate were used. The channel layer exhibited superconducting properties when its thickness was more than 9 nm, whereas channels less than 7.5 nm thick were in the insulating phase of the superconductor-insulator transition. The modulation ratio of the conductance of the insulating (I)-phase channels was a few orders of magnitude higher than the modulation ratio of the carrier density. The field-effect mobility increased with increasing channel thickness when the film was in the insulating phase. The sheet conductance of channels shorter than 10 &mgr;m was higher than that of a 100-&mgr;m-long channel of the same thickness due to a size effect on the transport of the I-phaseY0.9Pr0.1Ba2Cu3Oychannel. The field-effect mobility was also enhanced by the size effect. These results imply that the performance of possible field-effect transistors with high-temperature superconductors may be much improved by utilizing an I-phase channel whose length is short enough for the size effect to be evident. ©1997 American Institute of Physics.