In this work we report the alignment property of nematic liquid crystal with variable pretilt angle on surfaces treated by argon plasma. The scanning electron microscope (SEM) image indicates that nanoparticles are deposited on treated PI film. Groove-like aggregating structures are observed and the direction of aggregate tends to parallel to the ion beam direction. X-ray photoelectron spectroscopic (XPS) analysis confirms that -Fe2O3 nanoparticles were deposited on the surface of polyimide (PI) films. We also deduce that the maghemite is the main factor determining the pretilt angle while a competition exists between the homogeneous and homeotropic alignments.