Char-N is the part of the nitrogen bound in the fuel (fuel-N) that remained in the char matrix during biomass pyrolysis, which is the initial of combustion. Therefore, char-N is responsible for the emission of NOx and N2O during combustion. In this study, biomass char was produced by pyrolysis of rice straw (RS) and pine sawdust (PS) under Ar atmosphere at 500-900 degrees C. The N-functionalities and the surface area of the chars were characterized by X-ray photoelectron spectroscopy and Brunauer-Emmett-Teller analysis, respectively. Then, combustion experiments of biomass char were conducted in Ar/O-2 to explore the conversion of char-N and the proportion of fuel-N released as char-NO (char-NO/fuel-N). Besides, biomass char was additionally combusted in N-2/O-2 as comparison. Results showed that NO was the dominant N-containing product and lithe N2O was found. NO2 was not observed. Both NO and N2O were formed by direct oxidation of char-N. No obvious difference was observed during char combustion in N-2/O-2 and Ar/O-2 environments. With the increase in temperature, the NO concentration generally increased whereas the reverse was observed for char-NO/fuel-N. The concentration of NO clearly increased as the O-2 percentage increased, while NO/char-N and char-NO/fuel-N were not obviously affected. The heating time showed negligible influence on char-N conversion. The increase in the char mass led to dramatically decrease in the conversion of char-N and char-NO/fuel-N. However, no clear correlation could be revealed between the char-N conversion and the evolution of N-functionalities as well the surface area of the char.