Present work aims to make a comparative study of the use of fuel additives in straight vegetable oil (SVO) and preheated SVO on combustion and emission characteristics of CI engine. Two commercially available fuel additives for diesel referred as A1 and A2 were selected for this work. The physical properties of test fuels were measured and Sauter Mean diameter (SMD) is calculated to correlate atomization characteristics. The structural and chemical analysis of both additives has been done. The test results showed that peak pressure, heat release rate and NOx emission were found to be decreasing with increase in temperature for Karanj oil. The additive A1 significantly reduced NOx for both diesel and Karanj oil as fuels at all temperature condition. Both additives reduced the injector deposits drastically.