The structure of TiO 2 prepared by arc ion plating was changed by RF substrate bias from anatase (0V) through amorphous (-50V) to rutile (or rutile+anatase) (>-100V). This tendency was almost independent of arc current (50-80A). As insulating TiO 2 deposited on the chamber wall, irregular discharge occurred in the chamber. This irregular discharge was enhanced by the substrate bias so that the growth rate was decreased from 1 to 0.6nm/s and the substrate temperature was decreased from 350 o C to 150 o C.The oxygen content was increased with decreasing arc current, but the crystallinity at a low arc current of 50A was low. The decomposition efficiency of NO 2 gas by TiO 2 photocatalyst prepared at arc current of 60A was high, which was due to the low oxygen deficiency. In particular, anatase structure showed the highest efficiency. Therefore, it is said that structure, composition and crystallinity of the TiO 2 films affect the photocatalytic efficiency.