This work attempts to develop a correlation to predict thermal NOx formation in compression ignition (CI) engine when fuelled with diesel and biodiesel. Attention was focussed on both the fuel properties (cetane number, density) and engine design factors (bore, stroke, compression ratio, fuel injection timing) while developing correlation, ensuring that it is suitable for all CI engine fuels tested on both constant and variable speed engines. It was observed that NOx emissions predicted through the correlation is comparable with those of the measured ones. The general trend is observed to be the same for both the predicted and measured NOx emission. At loads more than 25% of the rated load the difference is found to be within 10% while the difference is slightly higher at loads less than 25%. It was also observed that the error was in the range of 10-20% at modified fuel injection timings for all the three esters at standard injection timing. This correlation yields comparatively accurate results for all the fuels at loads greater than 25% of the rated load and at standard fuel injection timing. (C) 2012 Elsevier Ltd. All rights reserved.