Besides triglycerides, Vietnam rubber seed oil contains a high amount of free fatty acids which make it dififcult to preserve and take part in a chemical modification. An esterification is one of many methods to reduce the acid value of the oils by converting the free fatty acids to methyl esters. Furthermore, this process do not change a composition of unsaturated componds in oils. It was found that the esterified oils have the acid value in the range of 47.6 to 1.12 mgKOH/g with different reaction time. Effect of the acid value of the oils on the epoxidation process is studied through a double bond conversion, an epoxidaiton yield and the oxirane content of an epoxidized oil. When the acid value of the oil reactant was above 4.98 mgKOH/g, the epoxidation yield increased with decreasing the acid value of the oil. However, when the acid value of the oil reactant was lower than 4.98 mgKOH/g, the epoxidation yield has almost no change if the acid value of the oil further declines.
