Alternative fuels for both spark ignition (SI) and compression ignition (CI) engines have become very important owing to increased environmental protection concern, the need to reduce dependency on petroleum and even socioeconomic aspects. An appropriate sustainable fuel alternative has turn out to be a main concern and bio-diesel is one of the sustainable fuels. The path of interest in biodiesel has highlighted its advantages which include decrease in hydrocarbon and particulate matter. Meanwhile its shortcoming includes higher emission of oxides of nitrogen. This work is an attempt to develop a mathematical relationship to predict thermal NOx in CI engine fuelled with neat biodiesel. Attention was focused on using in-cylinder pressure based variables to predict NOx. In cylinder pressure measurement is a valuable tool for the analysis of CI engine combustion, which is used for finding the heat release rate, ignition delay, etc. The parameter used for the NOx correlations are ignition delay, maximum temperature and density of the fuel. Mathematical correlation is developed from the experimental values of B-100 (Neat) biodiesel derived from Calophyllum Inophyllum oil. Experiments are conducted using biodiesel derived from non-edible oils to validate the correlated results. NOx emission predicted through correlation is compared with the biodiesel derived from B-100- Calophyllum Inophyllum biodiesel,B-30 elanji biodiesel (EME) and B-20 biodiesel prepared from waste cooking oil(WCME). The correlation developed is almost matching for B-100 Calophyllum Inophyllum.EME-B30 have a maximum error of 18.5%.WCME B20 have a maximum error of 23.5%.There was a significant reduction in error percentage of the developed correlation, as the biodiesel quantity in the blend increases.