Results are reported on the effect of fuel Lewis number (Le(F)) in turbulent jet diffusion flames of diluted H-2 fuels. The results include the flame temperature, the emission index of nitric oxides (EINOx), and the Damkohler number (Da) scaling of EINOx (EINOx/[L-f(3)/d(F)(2)U(F)] = Da(n), where L-f, d(F), and U-F are flame length, jet diameter, and jet exit velocity, respectively), which has been previously established. The effect of Le(F) was found to exist, although diminished in turbulent flames. Depending on the level and the type of dilution, the diluent that resulted in a lower value of Le(F) also had a higher flame temperature and produced larger values of both EINOx and the normalized emission index EINOx/[L-f(3)/d(F)(2)U(F)], which is also considered as the global NO formation rate of the flame. The exponent n = - 1/2 was reproduced for the present He-diluted flames whose Le(F) approximate to 1.0. For Ar- and CO2-diluted flames with Le(F) greater than or equal to 1.0 (20% and 40% dilution), n = -1/2 was also preserved. For dilution with 60% Ar or CO2, whose Le(F) are 0.68 and 0.528, respectively, the exponent n was found to be positive. These results suggest the significant effects of Le(F) on NOx emission from turbulent diffusion flames. (C) 2000 by The Combustion Institute.