Abstract: The esterification process of acidified oil with glycerol was thoroughly investigated by varying reaction temperature, molar ratio of glycerol to the free fatty acids in oil and the monoglyceride content in starting materials. It is found that monoglyceride can significantly promote the reaction process and lead to a much lower content of fatty acids in the product. This is elucidated by the additional analysis of experimental liquid-liquid equilibrium data for the binary systems of triglycerides-glycerin, oleic acid-glycerol and monoglyceride-glycerol, as well as the ternary phase diagram of glycerol-monoglyceride-oleic acid system. The comparison between the above ternary and binary systems clearly indicates that the appearance of monoglyceride remarkably improves the solubility of both glycerol and fatty acids in its counter phase, which can account for the observed promotion effect of monoglyceride in the esterification process of acidified oil.