The production of biodiesel from waste palm cooking oil using acidic ionic liquid as a catalyst was investigated. Generally, alkaline based catalysts are used to catalyze the transesterification reaction, but for waste cooking oil where it contains high free fatty acids, direct usage is not possible due to separation of layers and saponification problems. In this study, a two-step process i.e. esterification and transesterification was performed. The ionic liquid butyl-methyl imidazolium hydrogensulfate (BMIMHSO4) was found to be effective due to its longer side chain. The highest biodiesel Yield was obtained with 5 wt.% BMIMHSO4, methanol:oil of 15:1,60 min reaction time, at 160 degrees C, and agitation speed of 600 rpm, reduced the waste cooking oil acid value lower than 1.0 mg KOH/g. The second step of transesterification catalyzed by KOH at 60 degrees C, 1.0 wt.% and 60 min of reaction time. The final yield was 95.65 wt.%. The synthesized biodiesel was analysed by H-1 NMR, FTIRTGA and GC, and its physiochemical properties were determined by standard ASTM methods. (c) 2014 Elsevier Ltd. All rights reserved.