Abstract: Transesterification of sunflower oil (SFO) and waste cooking oil (WCO) with methanol was performed using CaO catalyst at 80-120°C under reflux conditions. The effects of the reaction temperature, the catalyst concentration and the molar ratio of methanol to oil on Fatty Acid Methyl Ester (FAME) yields were investigated. The FAME yield for transesterification of SFO indicated the maximum value at a molar ratio of 9:1 at 120°C. The water containing in WCO activated the CaO catalyst for transesterification. Therefore, the FAME yield for transesterification of WCO decreased as the molar ratio increased, due to the dilution of water by methanol. The FAME yields of SFO were higher than that of WCO under the same conditions, due to the presence of free fatty acids in WCO. At a reaction temperature of 80°C for 120 min with 3 wt.% CaO catalyst, the FAME yields of SFO and WCO reached more than 92 and 84%, respectively. The fuel properties of the biodiesel fuel (BDF) produced from SFO at 120°C with 5 wt.% CaO catalyst satisfied the values required in the EU standard for BDF (EN-14214).