Activated Mg-Al hydrotalcite (HT-Ca) nanoparticles (<45 nm) were synthesized by co-precipitation and hydrothermal activation with aqueous Ca(OH)(2) solution. They were characterized by various techniques including X-ray diffraction, inductively coupled plasma atomic-emission spectrometer, Brunauer-Emm ett-Teller method, scanning electronic microscope-X-ray energy dispersive analysis and temperature programmed desorption method. HT-Ca presented both acidic and basic due to the formation of Mg4Al2(OH)(14)center dot 3H(2)O, Mg2Al(OH)(7) and AlO(OH) nanocrystals to esterify and transesterify oils with high acid value (AV). Under conditions of 5 wt% HT-Ca, 160 degrees C, 30/1 methanol/oil molar ratio and 4 h, 93.4% Jatropha biodiesel yield was obtained at AV of 6.3 mg KOH/g with 4 cycles (biodiesel yield > 86%). It was further found that it can resist free fatty acids, and biodiesel yield reached 92.9% from soybean oil with high AV of 12.1. HT-Ca catalyst showed a potential practical application for direct production of biodiesel from oils with high AV without pretreatment. (C) 2015 Elsevier Ltd. All rights reserved.