Organic light emitting diodes (OLEDs) have the potential to compete with other rivals in applications of lighting and small size displays because of low power consumption. To achieve commercial standard, however, the luminance efficiency and device lifetime of the OLEDs still need to be improved. This work characterizes the performance of OLEDs improved by Poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS). PEDOT:PSS films are deposited on ITO glass by spin coating at the first, and then the hole transporting layer and electron transporting layer of the OLED materialsN,N'-bis (naphthalene-1-yl)-N,N'-bis (phenyl) benzidine/tris-(8-hydroxyquinoline) aluminum (NPB/Alq₃) are thermally evaporated with 1 Å/s deposition rate. The thickness effects of the PEDOT:PSS on the OLED performance are studied. The result shows that PEDOT:PSS effectively improves the luminance of the OLEDs, where 40 nm thick PEDOT:PSS increases 12% and 30 nm thick increases 31% luminance, respectively. Furthermore, the effects of the PEDOT:PSS on the mechanical properties of the OLED are also studied.