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Biodiesel production from waste-or by-product oils having high free fatty acid (FFA) contents is required oil pretreatment. Esterification has been used to reduce FFA contents of crude oils prior to transesterification. In this present work, palm empty fruit bunch (EFB) oil which is a by-product from the palm oil industry was used as feedstock for biodiesel production. The EFB oil has very high FFA contents. Thus, esterification of the oil was carried out. The esterification was enhanced by dual frequency ultrasound waves (28 kHz + 40 kHz). Zeolite was used as a heterogenous acid catalyst. Methanol to oil (M:O) molar ratio, catalyst loading, and reaction were main factors affecting on the process and were investigated using central composite design (CCD) of experiment. Response surface methodology (RSM) was used to optimise these parameters. The results showed that at the optimised condition of 12:1 M:O molar ratio, 11.36 %wt catalyst loading and 60 min of reaction time, the FFA conversion was highest at 91.69% compared with the predicted value of 93.56% which was only 1.99% difference between tested and predicted values.