After investigating the effects of each parameter on the esterification of FFA catalyzed by Amberlyst-15 in the trap grease, the optimal values were determined through an orthogonal experimental design. The optimum conditions for the esterification were determined to be 27:1 molar for the methanol to FFA in the trap grease, 95°C for the reaction temperature and 3 h for the reaction time. Stability of Amberlyst-15 was also studied under the optimal reaction conditions. The stability of the catalyst was not decreased much even after 10 times repeated use. The acid value was decreased from 100 mg KOH/g to 1.3 mg KOH/g in this process.