NOVELTY - Method for preparing calcium fatty acid and biodiesel, involves processing egg shell to obtain calcium oxide, adding the calcium oxide and illegal cooking oil to water, stirring the mixture, performing agglomeration, removing agglomerated calcium fatty acid from the mixture, filtering the mixture, evaporating the filtrate, purifying the calcium fatty acid, centrifuging the mixture, adding absolute methanol and the calcium oxide to the purified oil, mixing, refluxing and centrifuging the mixture, and performing distillation under reduced pressure.    USE - The method is useful for preparing calcium fatty acid and biodiesel.    ADVANTAGE - The method has mild process condition and high utilization ratio of the raw materials, consumes less time, is cost-effective and environmentally-friendly, and provides high yield of the biodiesel. The method utilizes illegal cooking oil and the waste eggshells in household garbage and hence waste is changed into treasure.    DETAILED DESCRIPTION - Method for preparing calcium fatty acid and biodiesel from eggshells and illegal cooking oil, involves (a) washing the egg shell using deionized water, drying the egg shell at 100 degrees C for 24 hours, grinding the dried shell, sieving the ground shell using sieve of 100 mesh, roasting the shell using muffle furnace at 900-1100 degrees C for 1.8-2.1 hours, where the heating rate is 10 degrees C/minute, and naturally cooling the heated shell to room temperature to obtain calcium oxide, (b) adding the calcium oxide and illegal cooking oil to water, where the mass ratio of illegal cooking oil to calcium oxide is 1:0.01-0.03, stirring the mixture at room temperature for 30-40 minutes at a speed of 700-800 revolutions per minute (rpm), heating the mixture at 75-80 degrees C, maintaining the mixture for 3-4 minutes, performing agglomeration, separating oil from the mixture, removing the agglomerated calcium fatty acid completely from the mixture by dissolving the mixture in tetrahydrofuran, filtering the mixture, removing the residual calcium oxide, evaporating the filtrate at 60-70 degrees C for 14-16 minutes to obtain the calcium fatty acid, purifying the calcium fatty acid, separating the oil using anhydrous magnesium sulfate and water, stirring the mixture at room temperature for 25-35 minutes, and centrifuging the mixture to remove the magnesium sulfate to obtain purified oil, (c) adding absolute methanol and the calcium oxide to the purified oil, where mass ratio of the purified oil, absolute methanol and calcium oxide is 1:0.33-0.6:0.03-0.06, stirring and uniformly mixing the mixture, refluxing the mixture at 65-70 degrees C for 3-4 hours, centrifuging the mixture separation and obtaining the supernatant, and performing distillation under reduced pressure of -0.1 MPa at 42-46 degrees C and to obtain biodiesel.