This paper presents a pre-mixed combustion model for diesel and Jatropha oil blends combustion studies. Jatropha oil blends are considered as a mixture of diesel and Jatropha oil. Computational Fluid dynamics (CFD) code FLUENT 6.3 is used for modeling the complex combustion phenomenon in compression ignition engine. The experiments are carried out on a single cylinder, four strokes, water cooled direct injection compression ignition engine at compression ratio of 17.5 at full load condition at constant speed of 1500 rpm fuelled with diesel and jatropha oil blends with diesel. The numerical model is solved by considering pressure based, implicit and unsteady solver and the effect of turbulence has taken into account. For turbulence modeling, Renormalization Group Theory (RNG) k-ε model is used. Sub models such as droplet collision model and Taylor Analogy Breakup (TAB) model are used for spray modelling. Species transport and pre-mixed combustion model are used for in-cylinder combustion. Computational results for a four stroke single cylinder diesel engine are comparing favourably against experiments. The results are in good agreement with experimental data. The present development yields a basis for detailed CFD studies of diesel and Jatropha oil blends combustion in a four stroke single cylinder diesel engine.be assigned by Global Digital Central and imprinted in your PDF file before the paper is published.