In this work, heterojunction light-emitting diodes made of Tb2O3 and poly(3,4-ethylene-dioxythiophene)-poly(styrene-sulfonate) (PEDOT:PSS) were prepared and the effect of PEDOT:PSS films on performance of the electroluminescence devices was studied. PEDOT:PSS films about 0, 50, 80, 120 and 150 nm were deposited on the 160 nm Tb2O3 film. Crystal structures, morphologies and electroluminescence properties of the devices were characterized, and the electroluminescence mechanism was analyzed. The turn-on voltages of the devices are almost the same under a forward bias. Four characteristic emission peaks of Tb3+ at 488, 544, 584 and 618 nm are observed from electroluminescence spectra of all the devices. The current decreases with increasing the thickness of the PEDOT:PSS film. However, the electroluminescence intensity first increases and then decreases under this condition. The electroluminescence intensity is up to 3.2 times higher than that of the pristine LED after spin-coating the PEDOT:PSS film. The optimum thickness of the PEDOT:PSS film is about 80 nm.