The requirement to significantly reduce NOx and particulate emissions while maintaining combustor performance is one of the main drivers for internal combustion engine research. Partially premixing and using fuel blends represent two promising approaches for reducing both the NOx and the particulate emissions from flames. This paper reports on the results of a numerical investigation on the effects of using different fuels on NOx emissions in counterflow partially premixed flames. The fuels investigated include methane, n-heptane, and their blends with hydrogen. The methane flame is computed using the GRI-3.0 mechanism, while the n-heptane flame is computed by combining the Held et al. oxidation mechanism with the Li and Williams NOx mechanism. Results indicate that, with regard to their NOx characteristics, partially premixed flames can be grouped into two distinct regimes, namely a double-flame regime, characterized by high levels of partial premixing and/or low strain rates (a(s)) with two physically separated reaction zones, and a merged-flame regime, characterized by low levels of partial premixing and/or high as with nearly merged reaction zones. In the first regime, NOx characteristics of both methane and n-heptane flames are strongly affected by changes in equivalence ratio (phi) and strain rate, while in the second regime, they exhibit a relatively weak dependence on phi and a(s). In addition, the n-heptane and methane flames established under identical conditions exhibit widely different NOx emission behavior in the first regime but qualitatively similar behavior in the second regime. Major differences include (i) significantly higher NO level and NOx emission index, (ii) much wider double-flame regime with regard to phi and as, (iii) dominance of the prompt mechanism over the thermal mechanism in the entire partially premixed regime, and (iv) noticeable reduction in NOx emission with hydrogen addition for n-heptane flames compared to methane flames. These differences are attributable to the different fuel pyrolysis/oxidation chemistry of the two fuels, as the consumption of n-heptane occurs mainly through the C-2 path, while that of methane occurs mainly through the C-1 path. As a result, the amounts Of C2H2 and, consequently, of CH radicals formed in n-heptane flames are significantly higher than those in methane flames and are responsible for the observed differences in NOx characteristics of the two fuels. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.