A central composite rotatable design was used to study the effect of methanol to oil ratio, reaction time, catalyst amount, and temperature on the transesterification of waste cooking palm oil using oil palm ash as a catalyst. The reaction was carried out at 10 bar. All of the variables except reaction time significantly affected the biodiesel yield, amount of catalyst and reaction temperature being the most effective, followed by methanol to oil ratio. Using response surface methodology, a quadratic polynomial equation was obtained for biodiesel yield by multiple regression analysis. The optimum conditions for transesterification of waste cooking palm oil to biodiesel were found as follows: amount of catalyst of 5.35 wt% (based on oil weight), temperature of 60 degrees C, methanol to oil ratio of 18.0 and reaction time of 0.5 h. The predicted and experimental biodiesel yields were found to be 60.07% (wt) and 71.74% (wt), respectively.