Transesterification of cottonseed oil and methanol with KOH as the catalyst for biodiesel production was carried out in microstructured reactors at residence times of less than 1 min and high flow rates. The reaction system included a micromixer that was connected to either a stainless steel capillary (inner diameter of 0.6 mm or a PTFE tube (inner diameter of 0.6 mm). packed with Dixon rings) where the reaction is supposed to take place. The influences of the type of the micromixer, the residence time, the methanol-to-oil molar ratio, the flow rate, the type of delay loop, and reaction temperatures below and above the boiling point of methanol were examined. Flow patterns Under different conditions were also examined in transparent microtubes. The results indicated that multilamination micromixers exhibited higher efficiencies in biodiesel production than simple T- and J-type micromixers. A higher yield of biodiesel could be obtained at reaction temperatures above the normal boiling point of methanol. By using the stainless steel capillary as the delay loop. the yield of biodiesel reached 94.8% Under the conditions of a methanol-to-oil molar ratio of 8:1. a flow rate of 2.5 mL/min, a residence time of 44 s, and a reaction temperature of 70 degrees C. Under such circumstances, the pressure drop was 0.8 MPa. By using the PTFE tube packed with Dixon rings as the delay loop, the yield of biodiesel reached 99.5% at the same methanol-to-oil molar ratio and reaction temperature, it flow rate of 10 mL/min, and a residence time of 17 s. The pressure drop was 0.7 MPa. Thus, very fast biodiesel production at high throughput can be achieved in microstructured reactors.