4-[(3-Methylphenyl)(phenyl)amino]benzoic acid (MPPBA) was synthesized in order to facilitate the hole-injection in Organic Light Emitting Diodes (OLED). MPPBA was applied to form self-assembled monolayer (SAM) on indium tin oxide (ITO) anode to align energy-level at the interface between organic semiconductor material (TPD) and inorganic anode (ITO) in OLED devices. The modified surface was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). KPFM was used to measure the surface potential and work function between the tip and the ITO surface modified by SAM technique using MPPBA. The OLED devices (ITO/MPPBA/TPD/Alq3/Al) fabricated with SAM-modified ITO substrates showed lower turn-on voltages and enhanced diode current compare to the OLED devices fabricated with bare ITO substrates.