Practical catalysts that work at a low temperature for selective catalytic reduction of NOx using NH3 (NH3-SCR) have been required to treat NOx at the outlet temperature in boiler systems (100-150 degrees C). In this paper, we report bulk vanadium oxide catalysts that show NH3-SCR activity at a low temperature below 150 degrees C. Defective bulk vanadium oxide (V(V)+V(IV)) catalysts were synthesized by the calcination of vanadium(IV)-oxalate at 270 degrees C (1-4 h). The reaction rate per mol of surface vanadium atom of the catalyst calcined at 270 degrees C for 2 h (V 270-2, 6.4 x 10(-2) mol(NO) molV(-1) s(-1)) was 10-14 times faster than those of conventional 1-9 wt % V2O5/TiO2 (4.5 x 10(-3)-6.1 x 10-3 mol(NO) mol(V)(-1) s(-1)), indicating that bulk vanadium oxide is more favorable for NH3-SCR and V(IV) species enhance the activity. The NH3-SCR of V 270-2 is driven by the Lewis acid mechanism, which proceeds faster than the Bronsted acid mechanism.