The two-stage continuous process of methyl ester from high free fatty acid (FFA) mixed crude palm oil (MCPO) was performed by using static mixer coupled with high-intensity of ultrasound. The 2 x 1000 W ultrasonic homogenizers were operated at 18 kHz frequency in the 2 x 100 mL continuous reactors. For the first-step, acid-catalyzed esterification was employed with 18 vol.% of methanol, 2.7 vol.% of sulfuric acid, 60 degrees C of temperature, and 20 L h(-1) of MCPO flow rate, for reducing the acid value from 28 mg KOH g(-1) to less than 2 mg KOH g(-1). For the second-step, base-catalyzed transesterification was carried out under 18 vol.% of methanol, 8 g KOH L-1 of oil, and 20 L h(-1) of esterified oil flow rate at 30 degrees C. The high yields of esterified oil and crude biodiesel were attained within the residence time of less than 20 s in the ultrasonic reactors. The yields of each stage process were: 103.3 vol.% of esterified oil, 105.4 vol.% of crude biodiesel, and 92.5 vol.% of biodiesel when compared with 100 vol.% MCPO. The quality of the biodiesel meets the specification of biodiesel standard in Thailand. (C) 2013 Elsevier Ltd. All rights reserved.