Four different enhancing methods including mechanical stirring (MS), flat plate ultrasonic irradiation (FPUI), fiat plate ultrasonic irradiation with mechanical stirring (UIMS) and probe ultrasonic irradiation (PUI) were studied to select a better one that need less catalyst, energy consumption and time to reach equilibrium for preparing biodiesel through transesterification of sunflower oil. The molar ratio of methanol to oil (3:1, 4:1, 5:1, 6:1, 7:1, 10:1 and 15:1), the catalyst concentration (0.5 wt.%, 1.0 wt.%, 1.5 wt.% and 2.0 wt.% of the weight of oil), the equilibrium time and energy consumption were studied. The PUI and UIMS methods reached the highest biodiesel conversion when methanol to oil ratio was 5:1 while that for MS was 6:1 and for FPUI was7:1. The suitable catalyst concentration for the reaction was 1.5 wt.%. At the same condition, the PUI and UIMS reached maximum biodiesel conversion about 25 min and that for MS and FPUI was about 50 min. The energy consumption of PUI and UIMS had no significant difference and were 0.19 kW h, 0.18 kW h, respectively. Results showed that under the same condition, UIMS and PUI used less catalyst, less methanol, shorter time and less energy consumption than MS and FPUI with the same biodiesel conversion. (C) 2011 Elsevier Ltd. All rights reserved.