This study investigated the transesterification of waste cooking oil (WCO) with methanol in the presence of potassium hydroxide as the catalyst. A semi-industrial pilot of microreactor with 50 tubes with a diameter of 0.8 mm was used to produce 5 L/h biodiesel. Initially, the acidity of waste cooking oil was reduced to less than 1 mg KOH/g oil by using methanol at 60 degrees C in the presence of 1% sulfuric acid. Using Box-Behnken design method, the effects of methanol to oil molar ratio (6:1-12:1), catalyst weight (0.5 -1.5 wt %), and reaction temperature (55-65 degrees C) were studied. The methanol to oil molar ratio of 9.4:1, the catalyst concentration of 1.16 wt %, and the reaction temperature of 62.4 degrees C was achieved under optimum condition. Finally, the effect of reaction time (30-120 s) was examined at the optimum condition. The highest level of biodiesel purity or fatty acid methyl esters % (FAMEs %) was 98.26%. In addition, the properties of produced biodiesel were determined and compared with those of the standard ASTM D6751. (C) 2019 Elsevier Ltd. All rights reserved.