Heterogeneous KF/CaO catalyst, prepared by the wet impregnation method with different KF loadings (15-35 wt.%) and calcination temperatures (600-1000 degrees C), was used to assist biodiesel production from microalgae Chlorella vulgaris under the combination of ultrasound and microwave (US-MW) irradiation. It was found that the catalyst calcinated at 900 degrees C with 25 wt.% KF loading presented the optimum performance, when the test was conducted with 12 wt.% of catalyst and a methanol to biomass ratio of 8:1 at 60 degrees C for 45 min. The highest fatty acid methyl esters (FAME) yield was 93.07 +/- 2.39%, which was significantly higher than that achieved by using US or MW irradiation individually. The catalysts were characterized by X-ray diffraction, scanning electron microscopy and Brunauer-Emmett-Teller. (C) 2014 Elsevier Ltd. All rights reserved.