Biomass and coal that contain high amounts of chlorine release HCl during combustion. There is no universal agreement regarding the effect of HCl on NO formation. To investigate the effect of HCl on NO formation, CO/NH3 combustion experiments and simulations were conducted in an entrained flow combustion reactor at 1023-1223 K. The experimental results show that the HCl addition tends to inhibit NO formation at low temperatures and exhibits promotion effects when the temperature is increasing and a temperature enhancement tends to inhibit NO formation. The simulation results provide evidence that the HCl addition inhibits NO formation by reducing the concentrations of the H, O, and OH radicals.