The main disadvantage of biodiesel is its high price. The price of biodiesel depends on various factors such as the price of oil, methanol, catalyst, and labor. Among dif-ferent economic factors, oil accounts for the largest share of input costs of biodiesel production. In this study, first, suitable heterogeneous catalysts were identified for biodiesel production. Several studies were carried out on biodiesel production using heterogeneous catalysts. All of these studies were designed to confirm that the pro-duction of biodiesel was cheaper than that of petroleum diesel. Waste materials as feedstock were used for this purpose. In transesterification reaction, waste cooking oil and waste materials were used as catalysts. Alkaline earth metal oxides catalysts are the best kind of heterogeneous catalysts. The catalytic reactivity of alkaline earth metal oxides including waste source of calcium oxide and magnesium oxide, CaO/Al2O3, CaO/SiO2, BaO/SiO2, and MgO/SiO2 were evaluated by the transesterification of oil and methanol. In this study, the costs of produced biodiesel were compared for different sources. The results indicated that the cost of produced biodiesel using synthetic catalysts was 1.26 to 1.49 times that using natural catalysts (1.26 and 1.49 are related to waste cooking oil and refined oil, respectively). Consequently, using waste cooking oils and natural catalysts is recommended for biodiesel production. Also, n-hexane as co-solvent was used to increase the solubility of methanol in oil. In presence of n-hexane, the cost of biodiesel production was approximately reduced by 16%
