To investigate the conversion of nitrogen bound in biomass fuel (abbreviated to fuel-N) during biomass pyrolysis, wheat straw (WS), rice straw (RS), spent coffee grounds (SCG) and palm kernel cake (PKC) were isothermally pyrolyzed in a horizontal tube reactor at the temperature range of 500-900 degrees C. The concentrations of light gaseous nitrogen containing species (gas-N) were measured online by a Fourier transform infrared (FTIR) spectroscopy gas analyzer, and the corresponding conversion rates were calculated. Results indicated that the conversion of fuel-N to gas-N as well as the evolution of N-functionalities in the portion of fuel-N that maintained in the char (char-N) were consistent regardless of the original N-functionalities in biomass samples. The conversion of fuel-N was found to be highly reliant on the temperature and fuel-N content. 17-47% of the fuel-N was finally retained in the char during pyrolysis. Therefore, the evolution of the N-functionalities from fuel-N to char-N was analyzed by X-ray photoelectron spectroscopy (XPS). Amide-N (N-A) was confirmed to be the dominant N-functionality in the raw biomass samples, and a small amount of pyrrolic-N (N-5), pyridinic-N (N-6) and quaternary-N (N-Q) were also identified. After pyrolysis, N-A was completely vanished in the char. However, only a very small fraction of N-A was decomposed into NH3 while most of it was preferentially converted to other gas-N (HCN, HNCO and NO) and N-5/N-6. For N-5/N-6, most of them preferred to retain in the char. As the temperature increased, a small amount of N-5/N-6 was converted to more stable N-Q and N-oxides (N-X) structures. Contrary to expected, N-A was also the main contributor to the formation of HCN, while the contribution from N-5/N-6 was less important. Finally, the conversion mechanism of fuel-N was concluded.