Biodiesel is considered as a renewable fuel and an alternative to petro diesel which is derived from transesterification of vegetable oils. In this research, the values of the major factors affecting the transesterification of sunflower oil were optimized by the response surface methodology. The experiments were conducted based on central composite rotatable design. A second order polynomial model was developed for predicting biodiesel yield as a function of the dependent variables. The optimum values obtained for molar ratio of oil to methanol, the percentage of catalyst (KOH), reaction time and reaction temperature were 1:5.5, 1.0%, 65.5 min and 51.7 degrees C, respectively. The yield of biodiesel using the optimized variables was 83.4%. The fuel tests indicated that the prepared biodiesel had good combustion characteristics and lower exhaust pollutant in compare with regular petro diesel. (C) 2012 Published by Elsevier Ltd.