Experimental studies were conducted to investigate the flame stability and the thermal/fuel NOx formation characteristics of the low calorific value (LCV) coal derived gas fuel. Synthetic LCV fuel gas was produced by mixing carbon monoxide, hydrogen and ammonia on the basis that the thermal input of the syngas fuel into a burner is identical to that of natural gas. The syngas mixture was fed to and burnt on flat flame burner. With the variation of the equivalence ratio for specific syngas fuel, flame behaviors were observed to identify the flame instability due to blow-off or flash-back and to define stable combustion range. Measurements of NOx content in exhaust gas were made to compare the thermal and the fuel NOx emissions from the LCV syngas combustion with those of the natural gas. In addition, the nitrogen dilution of the LCV syngas was attempted as an NOx reduction technique, and its effects on NOx emission and flame stability were investigated.