Transesterification of soybean oil with methanol in the presence of potassium hydroxide, as a catalyst, in a microreactor has been investigated. The transesterification reaction was performed at specific condition in circular tubes with hydraulic diameter of (0.8 mm). In order to further improve the biodiesel production, the experimental design was performed using Box-Behnken method. The results were analyzed using response surface methodology. The influence of reaction variables including; molar ratio of methanol to oil (6:1-12:1), temperature (55-65 degrees C) and catalyst concentration (0.6-1.8 wt.%) and residence time (20-180 s) under various flow rates of reactants (1-11 ml min(-1)) on Fatty Acid Methyl Ester (FAME) transesterification reaction was studied. The optimum condition was found at molar ratio of methanol to oil (9:1), catalyst concentration (1.2 wt.%) and temperature (60 degrees C) with a FAME % of about 89%. Considering optimum parameters, by changing the reactant residence time the FAME % was reached to 98% at 180 s. (C) 2014 Elsevier Ltd. All rights reserved.