Ethylene glycol dimethacrylate was synthesized by direct esterification of ethylene glycol and methacrylic acid. Instead of the water-carrying agent, air was used to remove the water that resulting from the esterification process. The effects of mixture gas of oxygen with nitrogen, argon and carbon dioxide respectively were studied. The results showed that nitrogen, argon and carbon dioxide have no assisted inhibit ability, and the assisted inhibit ability of the mixture of oxygen with other gases were better than pure oxygen. Air was the optimum water-carrying gas which can reduce the dosage of inhibitor. The effects of dosage of catalyst, ratio of ethylene glycol and methacrylic acid, reaction time, rate of air and dosage of inhibitor on yield were also investigated. The yield was 93.5% at the optimum conditions: dosage of tosic acid was 5% of the total weight of reactants, n(ethylene glycol):n(methacrylic acid) = 1:2.4, volume flow rate of air was 0.2L/min, dosage of hydroquinone was 0.1% of the total weight of reactants, reacting at 110℃ for 3 hours.