In the present study the effect of swirl on NOx formation in non-premixed propane –air flame is examined. The experiments are conducted in an ax symmetric cylindrical combustion chamber for a range of equivalence ratio and swirl angle. The numerical simulations are carried out with the CFD code, Fluent. The turbulence k - ε model is employed. Both predictions and experiments show that as the swirl number increase, the temperature and the thermal NOx first slightly increase and then strongly decrease at the exit of the combustion chamber. The predicted results are in good agreement with the measured results.