Factors acting on the deactivation of V2O5/TiO2 catalysts were investigated in the selective catalytic reduction (SCR) process for long term operation. The activity of V2O5/TiO2 catalysts was decreased rapidly after 8 months from the starting of operation in the selective catalytic reaction processes. From ICP-AES analysis, the deactivation of the used catalysts could be caused from the calcium component included in urea solution as a reducing agent. It was found from the NH3-TPD experiments that the strong basic element like Ca component drastically affected the acidity of the V2O5/TiO2 catalyst. The results gave an explanation on the reason why the component of Ca, even though its concentration is very low, could lead to the deactivation of V2O5/TiO2 catalyst in the selective catalytic reaction processes.