The activity of TiO2 in photo-assisted selective catalytic reduction of NO with NH3 (photo-SCR) was tested by using a flow reactor. The activity of TiO2 increased with increase in the amount of acid site. Density functional theory (DFT) calculations suggest that the rate of the NH2NO decomposition, which is the rate-determining step of the photo-SCR, on the weak acid site is faster than that on the strong acid site. The activity of the photo-SCR was enhanced by the addition of a small amount of WO3 on TiO2 and 1.5 wt.% WO3/TiO2 showed the highest activity among the tested. According to the NH3 adsorption experiments, the photo-SCR activity of WO3/TiO2, depends on the amount of the weak acid site. This result corresponds to the calculations by DFT. We concluded that the amount of the weak acid site on the catalyst is an important factor for the photo-SCR activity. (C) 2008 Elsevier B.V. All rights reserved.