The objective of this study was to investigate in transesterification study of linseed with methanol in the present of potassium hydroxide (KOH) for the production of biodiesel fuel. Experiments were conducted using milled linseeds, methanol as a solvent and potassium hydroxide as a catalyst. The influence of methyl alcohol: oil ratio (molar ratio) (6-12), catalyst concentration (.5-1.5 %wt. oil content), stirring rate (12000 rpm), reaction temperature (55-65 Ͼ⁰) and reaction time (90min) was examined to define optimum biodiesel yield and biodiesel quality after water washing and drying. The reaction time and stirring rate were fixed by preliminary tests. molar ratio, reaction temperature and catalyst concentration parameters were investigated by a set of experiments that designed by “Design Expert software (edition 11; Stat-Ease, Inc., USA) in response surface methodology (RSM) based box-Behnken design (BBD) method with duplication of each test due to improve confidence limit. In the long run, optimal conditions were: 0.92% of catalyst KOH, 8.68 for molar ratio, reaction temperature of 61.57 Ͼ⁰ , reaction time of 90 minutes, and agitation rate of 12000 rpm. In the lab scale and using these conditions, 94.1241% yield of biodiesel. Then applied this Biodiesel oil in diesel engine which Pour point and cloud point of the fuel has been measured to be 6.25 Ͼ⁰ and 3.17 Ͼ⁰  respectively Flash point 151 Ͼ⁰ Carbon residue 0.018%mass Ash content 0.0013 %mass The in-cylinder pressure and HRR were lower for linseed biodiesel compared with diesel fuel. This is ascribed to the low energy content and high latent heat of vaporization that cause a deterioration in the combustion process. The BTE reduced while the BSFC increased for the linseed biodiesel
compared with the diesel fuel. The smoke opacity, UHC, and CO. levels were lower for linseed biodiesel compared with diesel oil. The NOx level increased for the linseed biodiesel compared with diesel oil.