Biodiesel production from waste cooking oil using calcined Mg/Al HT (hydrotalcite) as heterogeneous catalyst was investigated. This study describes the calcined Mg/Al HT prepared under optimal conditions to catalyse waste cooking oil for biodiesel preparation and proposes a plausible catalysis mechanism. The catalysts were characterised by Fourier Transform-Infrared, X-ray diffraction, Thermal Gravity Analysis Differential thermal gravity and Brunner Emmet Teller measurements. Hydrotalcite with Mg/Al ratio of 3:1 showed a uniform mesoporous structure, excellent crystallinity, high surface area (270.5 m(2)/g) and good catalytic activity (at 500 degrees C calcination). The highest biodiesel yield obtained was 95.2% under optimised conditions of alcohol oil molar ratio of 6:1, methanol/ethanol molar ratio of 4:2, catalyst content of 1.5%, reaction time of 2.5 h, reaction temperature of 80 degrees C. Mixed methanol/ethanol showed good synergistic effects as an ester exchange agent, and the catalyst was easily separated and recycled. Therefore, Mg/Al hydrotalcite can effectively catalyse waste cooking oil for biodiesel preparation with mixed methanol/ethanol. (C) 2016 Published by Elsevier Ltd.