The ultrasonic-assisted transesterification of palm oil in the presence of alkaline earth metal oxide catalysts (CaO, SrO and BaO) was investigated. Batch process assisted by 20 kHz ultrasonic cavitation was carried out to study the effect of reaction time (10-60 min), alcohol to palm oil molar ratio (3: 1-15 :1), catalysts loading (0.5-3%) and varying of ultrasonic amplitudes (25-100%). The activities of the catalysts were mainly related to their basic strength. The catalytic activity was in the sequence of CaO < SrO < BaO. At optimum conditions, 60 min was required to achieve 95% yield compared to 2-4 h with conventional stirring. Also, the yields achieved in 60 min increased from 5.5% to 77.3% (CaO), 48.2% to 95.2% (SrO), and 67.3% to 95.2 (BaO). Fifty percentage amplitude of ultrasonic irradiation was deemed the most suitable value and physical changes on the catalysts after the ultrasonic-assisted reaction were successfully elucidated. BaO catalyst underwent relatively more severe activity drop in the catalyst reusability test. Catalysts dissolution was found to be mainly responsible for activity drop of the reused catalysts, especially with BaO catalyst. (C) 2010 Elsevier Ltd. All rights reserved.