Transesterification of sunflower oil with methanol to form fatty acid methyl esters (FAMEs) using a KOH catalyst was performed in a microtube reactor. Oil conversion was significantly affected by microtube length, microtube diameter, the methanol/oil molar ratio, and reaction temperature. Flow patterns were observed in the transparent microtube under different operating conditions and were characterized by optical measurements. The relationship between flow pattern and oil conversion was examined. The flow pattern at the entrance region of the microtube was segmented flow of the methanol and oil phases. As the reaction progressed, fine droplets composed of the produced glycerol and methanol were dispersed and circulated in the oil segments. At a methanol/oil molar ratio of 23.9 at 60 degrees C, a quasi-homogeneous phase formed approximately 300 mm from the reaction inlet where the oil was completely converted to FAMEs.