Water-soluble sulfonated poly(diphenylamine) (SPDPA) was synthesized and applied as a hole injection layer (HIL). It was characterized in terms of H-1 NMR, C-13 NMR, elemental analysis (EA), UV-vis, Raman, ultraviolet photoelectron spectroscopies (UPS) and TGA thermal stability measurement. The high transparency in visible region and acceptable conductivity were beneficial as HIL material. The topographic image obtained from Atomic Force Microscope shows that the Surface of SPDPA-coated indium tin oxide (ITO) is smoother than that of bare ITO. Raman spectra indicate that SPDPA possesses the higher oxidation and doping level due to SO3H group on the phenyl ring, rendering SPDPA the higher work function with 0.35 eV than PDPA from UPS spectra. The electroluminescence efficiency of polymer light emitting diode using poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) as an active layer and SPDPA as HIL can be reached 2.0 cd/A, showing the slightly better performance than that using PEDOT:PSS as HIL. (C) 2007 Elsevier Ltd. All rights reserved.