Transesterification of castor oil, sunflower oil and mixture of both oils were conducted in a millichannel reactor with 1.6 mm internal diameter. The flow behavior of the sample in millichannel reactor was observed. Both of the oil was mixed from 100 mL of castor oil until 20 mL of castor oil with total amount of 100 mL solution before used in the reaction. The alkali catalyst was mixed together with methanol before entering the reactor. In this study, the behavior of the flow pattern inside the millichannel reactor was observed under fixed parameters which are 21:1 methanol-to-oil molar ratio, 5.4 wt of KOH concentration in methanol, 180 s of residence time and 60 °C of temperature. The FFA value of both the oils was less than 2 mg-KOH/g-oil. The viscosity and density of the oils were determined before and after mixing the oils. The flow behaviour of the castor oil formed parallel flow throughout the reactor while sunflower oil formed slug flow in the reactor. When the volume of castor oil is reduced from 100 mL to 20 mL, the flow behaviours of the mixture oil were changed from slug flow at upstream to parallel flow at downstream. The formation of slug flow in the reactor improved the biodiesel yield: 88.4 of FAME yield. © 2016 The Authors.