We demonstrate improved performances in polymer light-emitting diodes (PLEDs) using a composite film of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and MoO3 powder as a hole injection layer. The PLED with the composite film exhibits the current efficiency of 13.5 cd/A, driving voltage of 3.4 V, and half lifetime of 108.1 h, while those values of the PLED with a pristine PEDOT: PSS was 11.3 cd/A, 3.8 V, and 41.5 h, respectively. We also analyze the morphological, optical and electrical properties of the composite films by atomic force microscopy (AFM), UV-Vis-IR absorption, and ultraviolet photoemission spectroscopy (UPS). This work suggests that mixing MoO3 into PEDOT: PSS is a simple and promising technique for use solution-based devices as an hole injection layer.