In this work, calcium oxide (CaO) extracted from eggshell impregnated with magnetite (Fe 3 O 4 ) is prepared successfully and it had been applied on transesterification of palm oil off-grade. Prior experiment, the eggshells material are powdered and calcined at 900 • C then impregnated with Fe 3 O 4 and recalcined. The obtained Fe 3 O 4 /CaO catalyst is characterized using X-ray diffraction and Braunaeur-Emmet-Teller (BET) surface area. The influence of various parameters including recalcined time and temperature are investigated. The prepared catalyst is tested for transesterification of palm oil off-grade to produce biodiesel in which the optimal conditions of a methanol/palm oil off-grade molar ratio of 10:1, the catalyst weight of 6%, the reaction temperature of 70 • C, and the reaction time of 2 h. The transesterification product was analyzed using GC-MS, which showed the biodiesel yield of 90% at the recalcined temperature of 600 • C and reaction time of 2 h. It has been noted that the catalyst activity is achieved when the moderate recalcination temperature is applied and the disordered structure of the catalyst is maintained. This study also confirms that CaO impregnated with Fe 3 O 4 could be a solid catalyst for the biodiesel synthesis through transesterification reaction of palm oil off-grade.Catalysts 2020, 10, 164 2 of 13 this issue, several countries have successfully implemented some renewables from bioenergy, including biogas, biodiesel, and bioethanol [2][3][4][5]. On the other sides, some countries have also successfully generated the electrical power from renewable energy resources, such as geothermal, waves, hydro, wind, and solar energy [6][7][8][9][10]. However, there are some weak points resulted while the renewable energy is used, such as it is unstable, only available for certain periods, and therefore, its required the energy storage devices such as battery. However, the energy storage capacity of batteries is very limited, and for this reason some scientists are attempting to discover another type of energy storage materials that can store significant amount of energy [11][12][13].Calcium is one of the most abundant metals on earth. It can be mostly found as an ion in seawater [14], as well as geothermal hot springs [15][16][17][18]. It can be also found as an oxide in limestone and fossilized remnant sites [14]. However, in order to obtain the calcium from these resources is required the natural resource exploitation that is often given negative impact to environment. Considering the environmental safety, an alternative way has been used to produce calcium source, namely it produce from some renewable resources which are available abundantly in nature, such as egg shell material wastes, because the egg shell well known as a natural material contained lots of calcium oxide [19]. As fact, the eggshell waste has been an abundant wastes in environment either come from egg consumption or the food industries. The waste materials are interesting to use as eco-friendly material to produce CaO ...