Abstract: The distribution of NO_x concentration in a 660 MW ultra supercritical opposed firing boiler was numerically simulated using Ansys fluent 14.0 software, of which the results were compared with actual measurements. Results show that the simulated results agree well with actual measurements, proving the simulation method to be effective. NO_x concentration in combustion zone of the first layer is higher than other two layers, which, at the combustor outlet, gets up to 0.001 rapidly; volumetric concentration of NO_x reduces gradually along the furnace height, starting from the combustor in the first layer; maximum NO_x concentration appears in the area near rear wall above combustion zone of the second layer.