Five reduced reaction mechanisms and a detailed mechanism have been incorporated into Monte Carlo simulation of NO formation in turbulent hydrogen jet flames with various amounts of helium dilution. Comparisons of the predicted NO emission indexes with recent experimental data show significant improvement in the predictions by using better reduced chemistry especially for high helium dilution cases. With proper time scales for chemistry and fluid dynamics, the experimentally observed NO, emissions are found to scale with the square root of the Damkohler number as EINOX = 0.09 {L(nu)(3)/(U(jet)d(jet)(*2))/tau(No)}(1/2) over four decades. This scaling relation is reproduced by the numerical model when better reduced chemistry is used.