Industrial kilns and power plants with high consumption of fossil fuels play a significant role in the production of air pollutants. Nitrogen oxide is one of these pollutants. In the present work, effect of different geometries on NO reduction in stack of industrial kilns and power plants is investigated numerically based on a selective non catalytic reduction (SNCR) method. In SNCR method, the NO reacts with ammonia which is injected into the kiln stack at temperature range of about 1150-1350 K and nitrogen is formed. In this study, a cylindrical stack with 500 cm length and 5 cm diameter is chosen similar to Ostberg experimental work. Four geometries for ammonia injection with one, two, four nozzles and by a ring around the stack have been studied. Numerical simulation of NO reduction by SNCR method shows that injection with one nozzle has lower efficiency than other injection geometries. Also effect of gas stack length on NO reduction has been investigated. The results show that increasing of stack length, has significant effect on ammonia slip reduction phenomenon. To investigate effect of ammonia injection nozzle angle on SNCR efficiency, nozzle angles between -75 to 75 degree were analyzed. Results show that the efficiency of this phenomenon decreases by increasing absolute value of injection nozzle angle. Finally, effect of baffle presence in mainstream has been studied. It is observed that and the required time and length for reaction decrease due to better mixing.