In this study, chemical parameters involved in waste olive oil transesterification were investigated and optimized. Better results were obtained using KOH and methanol instead of NaOH and ethanol, which decreases transesterification rates. The presence of KOH and methanol above or below the optimum quantity decreases the ester yield because of the presence of soaps or unreacted glycerides, respectively. Settling at ambient temperature under 25 degreesC increases the difficulty of ester and glycerol separation because of a conflict between glycerol solubility and low temperatures. This could be solved by increasing the settling temperature or the time for settling. In summary, the reaction was optimized at ambient temperature using 1.26% KOH, 12% methanol, 1 min of stirring, with 90 min of pour-off time, 11.38% distilled water by volume at 25 degreesC to purify the ester, and drying over 0.5% Na2SO4. Losses of esters during the washing process were less than 4%. The ester yield of the reaction was 94%. The small presence of unreacted glycerides did not drop the engine performance. Fuel specifications were close to those of diesel fuel, thus indicating that methyl esters from used olive oil can be considered as a fuel candidate.