The current work aims to develop synthesis biodiesel through simultaneous transesterification and ozonation reactions in a microtube reactor of 1 mm diameter, which has low energy consumption, provides process safety, as well as enables high reaction yield and conversion.The synthesis were carried out by contacting two immiscible liquids oil-methanol and ozone gas in a microtube of 5 m long at 30 °C, 1 atm pressure, the oil-methanol molar ratio 1:3, 1:5, with 1 wt% catalyst NaOH. For the purposes of comparison and assessment the influence of ozone gas, biodiesel was also synthesized by performing the reactions in series, i.e., transesterification then ozonation, and vice versa, by utilizing the same experimental conditions. After a preliminary study on gas-liquid-liquid flows, the result shows that the microtube reactor used is capable of producing a high yield and conversion through the simultaneous processes in a much shorter reaction time around 15 minutes by applying the lower ozone flow rates and the smaller methanol-oil molar ratio 1:3 than the previous works in stirred tank reactors. Ozone in this synthesis has broken the double bounds carbon chains of methyl oleate, methyl linoleate, and methyl linolenate and converting them to the saturated methyl esters, therefore, the biodiesel product has a better stability.