The effect of the methanol/oil mole ratio may be a key parameter due to the high MW absorption of methanol [18]. Owing to its high dielectric constant (ε ¼ 33) as compared to palm oil (ε ¼ 3), methanol strongly absorbs MW energy. The effect of the methanol/oil mole ratio on biodiesel yield is shown in Table 2 (System 1). It was found that the optimal methanol/oil mole ratio was 9. This was because transesterification is the reversible reaction and hence requires a higher amount of methanol to shift the reaction [10,17,18,19,21]. However, an excessive amount of methanol will also hinder the reaction rate because it would increase the solubility of the by-product glycerol, for the reverse reaction .The methanol/oil mole ratio does not significantly affect the outlet temperature, perhaps because of the boiling of methanol. At a high mole ratio, the amount of methanol is greater and more energy is needed to turn methanol into vapor and thus the temperature increase is lower.