Removal of naphthenic acids is very important for oil refineries to lower its corrosion of equipments. Esterification can be an interesting method to lower the acid concentration of high acid number oils. The kinetics of esterification of low-concentration naphthenic acids and methanol in oils with or without SnO as a catalyst have been investigated at 180, 200, 220, 250, and 280 degrees C in an autoclave with a methanol and naphthenic acid molecular ratio 10: 1. The experiment data follows a second-order reaction equation with or without SnO as a catalyst. The activation energy, "E-a", the frequency factor, "A", and reaction rate constant, "k", were also determined. The use of SnO can obviously lower the activation energy.