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The calculation of the Nitrogen Oxide (NO) formation emitted from diesel engines usually involve direct integration of a set of nitrogen chemistry elementary reactions that involve formation and destruction of NO. The primary hydrocarbon chemistry is usually simplified as long as the main species and heat release are predicted correctly. The result of the integration is the net NO formation rate evaluated using the local concentrations and thermodynamic parameters. In the present work a method for calculating NO emissions from diesel engines is proposed that takes into consideration the effect of residence time as a measure of turbulence effects on chemistry. This is based on the assumption that for mixing-limited conditions the turbulent eddy turn-over time can be taken as a characteristic reaction residence time. The proposed procedure depends on a detailed investigation of the primary hydrocarbon combustion chemistry decoupled from the flow-field prediction.