Short chain alcohol esters of fatty acids can be used as diesel fuel. In this study, one step and two step base catalyzed room temperature transesterification reaction of used cooking oil was compared. In the two step base catalyzed process, for 1000 g of used cooking oil 4.2 g NaOH and 140 ml MeOH was used in the first step and 1.8 g NaOH and 60 ml MeOH was used in the second step. All reactions were done at 25 degrees C; the effects of water content and suspended particles on the yield were studied. The yields were easily determined by Thermo Gravimetric Analysis (TGA) instead of the usual Gas chromatography (GC) and the viscosity of products was measured by Ubbelohde type viscosimeter. It was found that two step processes gives a better yield (96%) than the one step process (86%). (c) 2007 Elsevier B.V. All rights reserved.