The esterification reactions between the fatty acids with different alkyl chain length and the N-methyl diethanolamine were investigated. The reaction conditions were optimized by orthogonal experiments with lauric acid as materials. The effects of reaction temperature, catalyst dosage, molar ratio of raw materials and reflux temperature on the esterfication were examined, and the optimized reaction condition were determined as following: reaction temperature of 200 oC, catalyst dosage of 0.3%(mass ratio of catalyst/total raw materials), n(lauric acid) : n(N-methyl diethanolamine)=1.8:1, reflux temperature 70 oC. Under the optimized condition, the mass fraction of methyl diethanolamine lauric acid di-ester in the product was more than 98%. In addition, the conditions of esterification reactions between the fatty acids with different alkyl chain length and N-methyl diethanolamine were optimized. Under the optimized condition, the mass fraction of fatty acid diethanolamine di-ester in all of these products was greater than 98%. The products were characterized by IR and 1HNMR.