NO can be reduced by biomass char, which is catalyzed by inherent potassium in char. The isothermal reactions of char-NO and temperature-programmed desorption of reacted chars were studied in a fixed-bed reactor with original biomass char and acid-washed biomass char as samples. Experimental results showed that significant formation of C(O) occurred during the isothermal reactions and more C(O) was formed on original biomass char than on acid-washed biomass char. The formation of C(N) on both chars was negligible, and even inherent C(N) was consumed. The main product of char desorption was CO, with a small amount of CO2. Both the thermal desorption spectra of two chars showed two peaks during CO production and one peak during CO2 production with the increase in the temperature. However, the production of CO or CO2 in original biomass char was much higher than that in acid-washed biomass char. A catalytic mechanism of inherent potassium on the charNO reaction was proposed, with non-dissociative chemisorption of NO on K catalytic active sites being the rate-controlling step.