Efficient biodiesel conversion from waste cooking oil with high free fatty acids (FFAs) was achieved via a two-stage procedure (an acid-catalyzed esterification followed by an alkali-catalyzed transesterification) assisted by radio frequency (RF) heating. In the first stage, with only 8-min RF heating the acid number of the waste cooking oil was reduced from 68.2 to 1.64 mg KOH/g by reacting with 3.0% H(2)SO(4) (w/w, based on oil) and 0.8:1 methanol (weight ratio to waste oil). Then, in the second stage, the esterification product (primarily consisting of triglycerides and fatty acid methyl esters) reacted with 0.91% NaOH (w/w, based on triglycerides) and 14.2:1 methanol (molar ratio to triglycerides) under RF heating for 5 min, and an overall conversion rate of 98.8 +/- 0.1% was achieved. Response surface methodology was employed to evaluate the effects of RF heating time, H(2)SO(4) dose and methanol/oil weight ratio on the acid-catalyzed esterification. A significant positive interaction between RF heating time and H(2)SO(4) concentration on the esterification was observed. (C) 2010 Elsevier Ltd. All rights reserved.