A high pretilt angle is generated by hydrogenated amorphous carbon (a-C:H) thin films treated with an ion beam method. a-C:H thin films are deposited by remote plasma enhanced chemical vapor deposition and are modified by an Ar ion beam at an incidence angle of 45degrees using an Kaufman ion gun. Ion beam irradiation results in the decrease of the thickness of a-C:H thin films and the increase of the sp(2) fraction. The optical transmittance of the ion beam irradiated a-C:H is more affected by thickness than sp(2) fraction. An excessive ion beam irradiation, 5 min Ar ion beam, results in the increase of the surface roughness of the a-C:H thin films and the pretilt angle has very low value. The pretilt angle is controlled by the thickness, sp(2) fraction and the surface roughness.