The NOx emission characteristics of CH4-air coflow jet flames were investigated experimentally and numerically. NOx was measured using a chemiluminescent detection method and calculated using a parabolic-type solver for conservation equations and a GRI-2.11 reaction mechanism for NOx chemistry. The fuel flowrate (Q(F)) diameter of the mixture nozzle (D-M), and equivalence ratio (Phi) were varied to investigate NOx emission characteristics. The emission levels from various flames were quantified in terms of the NOx emission index (EINOx), a dimensionless quantity. The trenlds in the predicted total EINOx with varying Phi, Q(F), and D-M were in good agreement with experimental values and our prediction method should be useful for investigating the NOx formation characteristics, while reducing calculation time. Prompt EINOx increased rapidly up to Phi = 1.5; it then increased linearly with Phi and was nearly indifferent to variation in Q(F). Thermal EINOx increased rapidly, reached a maximum value at Phi = 1.5, and then decreased slowly. Moreover, it increased with increasing Q(F). By contrast, the thermal and prompt EINOx were nearly indifferent to variation in D-M. The measured EINOx and predicted total EINOx had similar trends with the variation in Phi, Q(F), and D-M. These EINOx trends were explained well by combining the trends in the prompt and thermal EINOx for differences in Phi, Q(F), and D-M. In coflow jet flames, the dominant NOx formation route can differ markedly according to Phi and Q(F); therefore, it should be described using the appropriate coupling of the equivalence ratio and fuel flowrate. (C) 2004 Elsevier Ltd. All rights reserved.